PHYSIOLOGY OF PREGNANCY. LABOUR AND POSTPARTUM PERIOD. THE PERINATAL CARE OF THE FETUS. METHODS OF PREGNANT INSPECTION/ PHARMACOTHERAPY IN OBSTETRICS
Prepared by N. Bahnij
The Placenta and Fetal Membranes
The development of the human placenta is as uniquely intriguing as the embryology of the fetus. The placenta is a fascinating organ, especially when its function is considered. During its brief intrauterine existence, the fetus is dependent upon the placenta as its lung, liver, and kidneys. The organ serves these purposes until sufficient maturation of the fetus allows it to survive ex utero as an air-breathing organism.
Despite its unassailable role in human fetal development, study of the placenta has lagged behind that of the fetus. A number of anatomists and embryologists worked through the 1980s to provide some basic knowledge. It has not been until recently that clinicians have appreciated the plethora of knowledge that can be gained by microscopic study of the placenta. This latter enlightenment has transposed through the efforts of placental pathologists such as Benirschke, Driscoll, Fox, and Naeye. Their work, as well as that of many of their colleagues, has shown that careful examination of the placenta may frequently shed light on the etiopatho-genesis of a number of maternal-fetal disorders (Benirschke, 2000; Benirschke and Kauffman, 2000). Abnormal placentation, placental pathology, and their effects on pregnancy outcome, as well as adverse placental effects from maternal diseases, are considered in Chapters 31 and 32.
Boyd and Hamilton (1970) presented a marvelous account of the history of placental research. A summary of this history was presented in Chapter 5 (p. 95) of the 20th edition of Williams Obstetrics. The interested reader is referred to this summary or to the treatise of Boyd and Hamilton (1970).
FETAL TISSUES OF THE FETAL-MATERNAL COMMUNICATION SYSTEM
The two arms of the fetal-maternal communication system of human pregnancy were described in Chapters 2 and 4 (see Fig. 5-2). The extravillous and villous trophoblasts are the embryonic-fetal tissues of the anatomical interface of the placental arm; the avascular fetal membranes—the amnion and chorion laeve—are the fetal tissues of the anatomical interface of the paracrine arm of this system.
The placental arm of this system links the mother and fetus as follows: maternal blood (spurting out of the uteroplacental vessels) directly bathes the syncytiotrophoblast, the outer surface of the trophoblastic villi; fetal blood is contained within fetal capillaries, which traverse within the intravillous spaces of the villi. This is a hemochorioendothelial type of placenta. The paracrine arm of this system links the mother and fetus through the anatomical and biochemical juxtaposition of (extraembryonic) chorion laeve and (maternal uterine) decidua parietalis tissue.
Therefore, at all sites of direct cell-to-cell contact, maternal tissues (decidua and blood) are juxtaposed to extraembryonic cells (trophoblasts) and not to embryonic cells or fetal blood. This is an extraordinarily important arrangement for communication between fetus and mother and for maternal (immunological) acceptance of the conceptus.
The role of the placenta in nidation and in the transfer of nutrients from mother to embryo-fetus has longed fueled interest in this unique organ. Subsequently, the enormous diversity of form and function of the placenta was recognized as the incredible metabolic, endocrine, and immunological properties of its trophoblasts were discovered.
EARLY HUMAN DEVELOPMENT
The definitions that follow are taken from Moore (1973, 1988).
• Zygote: The cell that results from the fertilization of the ovum by a spermatozoan.
• Blastomeres: Mitotic division of the zygote (cleavage) yields daughter cells called blastomeres.
• Morula: The solid ball of cells formed by 16 or so blastomeres.
• Blastocyst: After the morula reaches the uterus, a fluid-filled cavity is formed, converting the morula to a blastocyst.
• Embryo: The embryo-forming cells, grouped together as an inner cell mass, give rise to the embryo, which usually is so designated when the bilaminar embryonic disc forms. The embryonic period extends until the end of the seventh week, at which time the major structures are present.
• Fetus: After the embryonic period, the developing conceptus is referred to as the fetus.
• Conceptus: This term is used to refer to all tissue products of conception—embryo (fetus), fetal membranes, and placenta. The conceptus includes all tissues, both embryonic and extraembryonic, that develop from the zygote.
FERTILIZATION OF THE OVUM AND CLEAVAGE OF THE ZYGOTE
Few, if any, naturally occurring phenomena are of greater importance to humankind than the union of egg and sperm. Fertilization occurs in the fallopian tube; and it is generally agreed that fertilization of the ovum must occur within minutes or no more than a few hours after ovulation. Consequently, spermatozoa must be present in the fallopian tube at the time of ovulation. Most all pregnancies occur when intercourse occurs during the 2 days preceding or on the day of ovulation. If intercourse takes place on the day after ovulation, pregnancy probably will not result.
Spermatozoa around ovum
After fertilization in the fallopian tube, the mature ovum becomes a zygote—a diploid cell with 46 chromosomes—that then undergoes segmentation, or cleavage, into blastomeres. The first typical mitotic division of the segmentation nucleus of the zygote results in the formation of two blastomeres (Fig. 5-1). The zygote undergoes slow cleavage for 3 days while still within the fallopian tube; fertilized human ova that are recovered from the uterine cavity may be composed of only 12 to 16 blastomeres. As the blastomeres continue to divide, a solid mulberry-like ball of cells, referred to as the morula, is produced. The morula enters the uterine cavity about 3 days after fertilization. The gradual accumulation of fluid between blastomeres within the morula results in the formation of the blastocyst (Fig. 5-2). The compact mass of cells at one pole of the blastocyst, called the inner cell mass, is destined to be the embryo. The outer mass of cells is destined to be the trophoblasts.
THE EARLY HUMAN ZYGOTE. Hertig and co-workers (1954) found in the two-cell zygote that the blastomeres and the polar body are free in the perivitelline fluid and are surrounded by a thick zona pellucida (Fig. 5-1A). In a 58-cell blastocyst, the outer cells which are progenitors of the trophoblasts can be distinguished from the inner cells that form the embryo (Fig. 5-2B). The 107-cell blastocyst was found to be no larger than the earlier cleavage stages, despite the accumulated fluid (Fig. 5-1C). It measured 0.153 by 0.155 mm in diameter before fixation and after the disappearance of the zona pellucida. The eight formative or embryo-producing cells were surrounded by 99 trophoblastic cells.
Just before implantation, the zona pellucida disappears and the blastocyst touches the endometrial surface; at this time of apposition, the blastocyst is composed of 107 to 256 cells. The blastocyst adheres to the endometrial epithelium, and implantation occurs most commonly on the endometrium of the upper part and on the posterior wall of the uterus. After gentle erosion between epithelial cells of the surface endometrium, the invading trophoblasts burrow deeper into the endometrium, and the blastocyst becomes totally encased within the endometrium, being covered over by the endometrium.
BIOLOGY OF THE TROPHOBLAST
Of all placental components, the trophoblast is the most variable in structure, function, and development. Its invasiveness provides for attachment of the blastocyst to the decidua of the uterine cavity; its role in nutrition of the conceptus is reflected in its name; and its function as an endocrine organ in human pregnancy is essential to maternal physiological adaptations and to the maintenance of pregnancy.
DIFFERENTIATION. Morphologically, trophoblasts are either cellular or syncytial, and may appear as uni-nuclear cells or multinuclear giant cells. At implantation, some of the innermost cytotrophoblasts or Langhans cells that are contiguous with and invading the endometrium, coalesce to become an amorphous, multinucleated, continuous membrane that is uninterrupted by intercellular spaces, the syncytium. There are no individual cells, only a continuous lining; therefore it is the singular "syncytiotrophoblast" or syncytium. The true syncytial nature of the human syncytiotrophoblast has been confirmed by electron microscopy. The mechanism of syncytial growth, however, was a mystery in view of the discrepancy between an increase in the number of nuclei in the syncytiotrophoblast and equivocal evidence (at best) of intrinsic nuclear replication. Mitotic figures are completely absent from the syncytium, being confined to the cytotrophoblasts.
FORMATION OF THE SYNCYTIUM. Ulloa-Aguirre and co-workers (1987) elegantly demonstrated the conversion of cytotrophoblasts to a morphologically and functionally characteristic syncytium in vitro. They established that at least part of this differentiation process involves the action of cyclic adenosine monophosphate (cAMP). Based on their methods of isolation and characterization of human cytotrophoblasts, others developed systems to evaluate blastocyst implantation in vitro (Kliman and associates, 1986; Ringler and Strauss, 1990). Isolated cytotrophoblasts, placed in serum-containing medium, migrate toward one another and form aggregates. Ultimately, the aggregates fuse and syncytium is produced during 3 to 4 days. The syncytium also is formed in the absence of serum, provided that extracellular matrix components are present to serve as a lattice for cytotrophoblast migration. The syncytium produced in vitro is covered by microvilli, as it is in vivo. Cytotrophoblast aggregation is dependent upon protein synthesis, and involves a calcium-dependent cell adhesion molecule, E-cadherin, for aggregation. Desmosomes develop between the cells; and as the cytotrophoblasts fuse, the expression of E-cadherin diminishes.
The cytotrophoblast is the germinal cell; the syncytium, or the secretory component, is derived from cytotrophoblasts. Therefore, the cytotrophoblasts are the cellular progenitors of the syncytiotrophoblast. Well-demarcated borders and a single, distinct nucleus characterize each cytotrophoblast; and there are frequent mitoses among the cytotrophoblasts. These characteristics are lacking, however, in the syncytium, in which the cytoplasm is amorphous, without cell borders, and the nuclei are multiple and diverse in size and shape. The absence of cell borders in the syncytium obliges transport across this structure. Hence, the control of transport is not dependent on the participation of individual cells.
Coutifaris and Coukos (1994) presented a succinct and informative review of the processes of human implantation. They point out that after apposition and adherence of the trophectoderm of the blastocyst to the endometrial epithelial cells, implantation commences by intrusion of cytotrophoblasts between endometrial epithelial cells. This process of trophoblast invasion is facilitated by degradation of the extracellular matrix of the endometrium/decidua, catalyzed by urokinase-type plasminogen activator, urokinase plasminogen activator receptor, and metalloproteinases that are produced by selected cytotrophoblasts at various stages of implantation/placentation. These functions of cytotrophoblasts invading the endometrium are indistinguishable from those of metastasizing malignant cells. As the cytotrophoblasts move through the decidua, selected populations of these cells bind to various extracellular matrix components of the decidual stromal cells. This facilitates migration and thence the establishment of placental anchors to the decidua.
IMMUNOLOGICAL ACCEPTANCE OF THE CONCEPTUS
Over the last half century, many attempts to explain the survival of the semiallogenic fetal graft have been proposed. One of the earliest explanations was based on the theory of antigenic immaturity of the embryo-fetus. This was disproved by Billingham (1964) who showed that transplantation (HLA) antigens are demonstrable very early in embryonic life. The trophoblasts are the only cells of the conceptus in direct contact with maternal tissues or blood and these tissues are genetically identical with fetal tissues. Another explanation was based on diminished immunological responsiveness of the pregnant woman. There is, however, no evidence for this to be other than an ancillary factor. In a third explanation, the uterus (decidua) is proposed as an immunologically privileged tissue site. Clearly, transplantation immunity can be evoked and expressed in the uterus as in other tissues. Therefore, the acceptance and the survival of the conceptus in the maternal uterus must be attributed to an immunological peculiarity of the trophoblasts, not the decidua.
CURRENT STATUS OF RESEARCH
It still is enigmatic that maternal tissues accept and tolerate the grafted conceptus. Moreover, the placenta likely expresses "novel" genes (Dizon-Townson and colleagues, 2000). Several novel aspects of the expression of the HLA system in trophoblasts, together with a unique set of lymphocytes, may provide an explanation for this.
IMPLANTATION AND INTEGRIN SWITCHING.
The attachment of the trophectoderm of the blastocyst to the endometrial surface by apposition and adherence and then the intrusion and invasion of the endometrium/decidua by cytotrophoblasts (implantation) appears to be dependent upon two factors:
1. Trophoblast elaboration of specific proteinases that degrade selected extracellular matrix proteins of the endometrium/decidua.
2. A coordinated and alternating process referred to as integrin switching, which facilitates migration and then attachment of trophoblasts in the decidua.
The integrins, one of four families of cell adhesion molecules (CAMs), are cell-surface receptors that mediate the adhesion of cells to extracellular matrix proteins (Frenette and Wagner, 1996). Great diversity of cell binding to a host of different extracellular matrix proteins is possible by way of the integrin system.
Recall that the decidual cell becomes completely encased by a pericellular (extracellular matrix) membrane. This "wall" around the decidual cell provides the scaffolding for the attachment of the extravillous trophoblasts, the anchoring cytotrophoblasts. These cells first elaborate selected proteinases that degrade the extracellular matrix of decidua. Thereafter, the expression of a specific group of integrins enables the docking of these cells. By alternating between these two processes and by "integrin switching," the movement of cytotrophoblasts into the decidua is aggressive, but regulated. Specific decidual localization of the cytotrophoblasts to establish attachment of the placenta to the wall of the uterine cavity results. Craven and colleagues (2000) have provided evidence that a similar process is operative for trophoblastic invasion of uterine veins.
ORGANIZATION OF PLACENTA
From the electron microscopic studies of Wislocki and Dempsey (1955), data were provided that permitted a functional interpretation of the fine structure of the placenta. There are prominent microvilli on the syncytial surface, corresponding to the "brush border" described by light microscopy Associated pinocytotic vacuoles and vesicles are related to the absorptive and secretory placental functions. The inner layer of the villi—the cytotrophoblasts—persists to term, although often compressed against the trophoblastic basal lamina, and retains its ultrastructural simplicity
Villi can first be distinguished easily in the human placenta on about the 12th day after fertilization. When a mesenchymal cord, presumably derived from cytotrophoblasts, invades the solid trophoblast column secondary villi are formed. After angiogenesis occurs from the mesenchymal cores in situ, the resulting villi are termed tertiary. Maternal venous sinuses are tapped early in the implantation process, but until the 14th or 15th day after fertilization, maternal arterial blood does not enter the intervillous space. By about the 17th day, fetal blood vessels are functional, and a placental circulation is established. The fetal-placental circulation is completed when the blood vessels of the embryo are connected with the chorionic blood vessels. Some villi, in which failure of angiogenesis results in a lack of circulation, become distended with fluid and form vesicles. A striking exaggeration of this process is characteristic of the development of hydatidiform mole
Proliferation of cellular cytotrophoblasts at the tips of the villi produces the trophoblastic cell columns, which are not invaded by fetal mesenchyme but are anchored to the decidua at the basal plate. Thus, the floor of the intervillous space (maternal-facing side) consists of cytotrophoblasts from the cell columns, the peripheral syncytium of the trophoblastic shell, and decidua of the basal plate. The floor of the chorionic plate, consisting of the two layers of trophoblasts externally and fibrous mesoderm internally, forms the roof of the intervillous space.
In early pregnancy, the villi are distributed over the entire periphery of the chorionic membrane. A blastocyst dislodged from the endometrium at this stage of development appears shaggy (Fig. 5-9). The villi in contact with the decidua basalis proliferate to form the leafy chorion, or chorion frondosum, the fetal component of the placenta; the villi in contact with the decidua capsularis cease to grow and degenerate to form the chorion laeve. The chorion laeve is generally more nearly translucent than the amnion even though rarely exceeding 1 mm in thickness. The chorion laeve contains ghost villi, and decidua clings to its surface.
Until near the end of the third month, the chorion laeve is separated from the amnion by the exocoelomic cavity. Thereafter, the amnion and chorion are in intimate contact. In the human, the chorion laeve and amnion form an avascular amniochorion, but these two structures are important sites of molecular transfer and metabolic activity. They constitute the paracrine arm of the fetal-maternal communication system.
Certain villi of the chor-ion frondosum extend from the chorionic plate to the decidua and serve as anchoring villi. Most villi, however, arboresce and end freely in the intervillous space without reaching the decidua (Fig. 5-10). As the placenta matures, the short, thick, early stem villi branch repeatedly, forming progressively finer subdivisions and greater numbers of increasingly small villi (Fig. 5-11). Each of the main stem (truncal) villi and their ramifications (rami) constitute a placental cotyledon (lobe). Each cotyledon is supplied with a branch (truncal) of the chorionic artery; and for each cotyledon, there is a vein, constituting a 1:1:1 ratio of artery to vein to cotyledon.
BREAKS IN THE PLACENTAL "BARRIER"
The placenta does not maintain absolute integrity of the fetal and maternal circulations. This is evidenced by numerous findings of the passage of cells between mother and fetus in both directions. This situation is best exemplified clinically by erythrocyte D-antigen isoimmunization and the occurrence of erythroblastosis fetalis (Chap. 39, p. 1061). Typically, a few fetal blood cells are found in maternal blood; but on extremely rare occasions, the fetus exsanguinates into the maternal circulation. Fetal leukocytes may replicate in the mother and leukocytes bearing a Y chromosome have been identified in women for up to 5 years after giving birth to a son (Ciaranfi and colleagues, 1977). Desai and Creger (1963) labeled maternal leukocytes and platelets with atabrine and found that the atabrine-labeled cells crossed the placenta from mother to fetus.
PLACENTAL SIZE AND WEIGHT
Crawford (1959) suggested that the total number of cotyledons remains the same throughout gestation. Individual cotyledons continue to grow, although less actively in the final weeks. Placental weights vary considerably, depending upon how the placenta is prepared. If the fetal membranes and most of the cord are left attached and adherent maternal blood clot is not removed, the weight may be greater by nearly 50 percent (Thomson and co-workers, 1969).
THE PLACENTA AT TERM
According to Boyd and Hamilton (1970), the placenta at term is, on average, 185 mm in diameter and 23 mm in thickness, with an average volume of 497 mL, and weight of 508 g; but these measurements vary widely. There are multiple shapes and forms of the human placenta and a variety of types of umbilical cord insertions, which are discussed in Chapter 32. Viewed from the maternal surface, the number of slightly elevated convex areas called lobes (or if small, lobules) varies from 10 to 38. These lobes are separated, albeit incompletely, by grooves of variable depth, the placental septa. The lobes are also referred to as cotyledons.
Maternal side of placenta
Fetal side of placenta
As the villi continue to branch and the terminal ramifications become more numerous and smaller, the volume and prominence of cytotrophoblasts decrease. As the syncytium thins and forms knots, the vessels become more prominent and lie closer to the surface. The stroma of the villi also exhibits changes associated with aging. In placentas of early pregnancy, the branching connective tissue cells are separated by an abundant loose intercellular matrix. Later, the stroma becomes denser and the cells more spindly and more closely packed.
Another change in the stroma involves the Hofbauer cells, which likely are fetal macrophages. These cells are nearly round with vesicular, often eccentric nuclei and very granular or vacuolated cytoplasm. These cells are characterized histochemically by intracytoplasmic lipid and are readily distinguished from plasma cells.
Certain of the histological changes that accompany placental growth and aging are suggestive of an increase in the efficiency of transport and exchange to meet increasing fetal metabolic requirements. Among these changes are a decrease in thickness of the syncytium, partial reduction of cytotrophoblastic cells, decrease in the stroma, and an increase in the number of capillaries and the approximation of these vessels to the syncytial surface. By 4 months, the apparent continuity of the cytotrophoblasts is broken, and the syncytium forms knots on the more numerous smaller villi. At term, the covering of the villi may be focally reduced to a thin layer of syncytium with minimal connective tissue; and the fetal capillaries seem to abut the trophoblast. The villous stroma, Hofbauer cells, and cytotrophoblasts are markedly reduced, and the villi appear filled with thin-walled capillaries.
Other changes, however, are suggestive of a decrease in the efficiency for placental exchange. These changes include thickening of the basement membranes of the trophoblast capillaries, obliteration of certain fetal vessels, and fibrin deposition on the surface of the villi in the basal and chorionic plates as well as elsewhere in the intervillous space.
The amnion at term is a tough and tenacious but pliable membrane. It is the innermost fetal membrane and is contiguous with the aminonic fluid. This particular avascular structure occupies a role of incredible importance in human pregnancy. In many obstetrical populations, preterm premature rupture of the fetal membranes is the single most common antecedent of preterm delivery. The amnion is the tissue that provides almost all of the tensile strength of the fetal membranes. Therefore, the development of the component(s) of the amnion that protects against rupture or tearing is vitally important to successful pregnancy outcome.
Bourne (1962) described five separate layers of amnion tissue. The inner surface, which is bathed by the amnionic fluid, is an uninterrupted, single layer of cuboidal epithelial cells, believed to be derived from embryonic ectoderm. This epithelium is attached firmly to a distinct basement membrane that is connected to the acellular compact layer, which is composed primarily of interstitial collagens I, III, and V. On the outer side of the compact layer, there is a row of fibroblast-like mesenchymal cells (which are widely dispersed at term). These cells are probably derived from mesoderm of the embryonic disc. There also are a few fetal macrophages in the amnion. The outermost layer of amnion is the relatively acellular zona spongiosa, which is contiguous with the second fetal membrane, the chorion laeve. The important "missing" elements of human amnion are smooth muscle cells, nerves, lymphatics, and importantly, blood vessels.
Early in the process of implantation, a space develops between the embryonic cell mass and adjacent trophoblasts (Fig. 5-5). Small cells that line this inner surface of trophoblasts have been called amniogenic cells, the precursors of the amnionic epithelium. The human amnion is first identifiable about the seventh or eighth day of embryo development. Initially, a minute vesicle (Fig. 5-5), the amnion, develops into a small sac that covers the dorsal surface of the embryo. As the amnion enlarges, it gradually engulfs the growing embryo, which prolapses into its cavity (Benirschke and Kaufman, 2000).
Distension of the amnionic sac eventually brings it into contact with the interior surface of the chorion laeve. Apposition of the mesoblasts of chorion laeve and amnion near the end of the first trimester then causes an obliteration of the extraembryonic coelom. The amnion and chorion laeve, though slightly adherent, are never intimately connected, and usually can be separated easily, even at term.
The amnion is clearly more than a simple avascular membrane that functions to contain amnionic fluid. It is metabolically active, involved in solute and water transport to maintain amnionic fluid homeostasis, and produces a variety of interesting bioactive compounds, including vasoactive peptides, growth factors, and cytokines.
The normally clear fluid that collects within the amnionic cavity increases in quantity as pregnancy progresses until near term, when there is a decrease in amnionic fluid volume in many normal pregnancies. An average volume of about 1000 mL is found at term, although this may vary widely from a few milliliters to many liters in abnormal conditions (oligohydramnios and polyhydramnios or hydramnios).
In early pregnancy, amnionic fluid is an ultrafiltrate of maternal plasma. By the beginning of the second trimester, it consists largely of extracellular fluid which diffuses through the fetal skin, and thus reflects the composition of fetal plasma (Gilbert and Brace, 1993). After 20 weeks, however, the cornification of fetal skin prevents this diffusion and amnionic fluid is composed largely of fetal urine. The fetal kidneys start producing urine at 12 weeks' gestation, and by 18 weeks are producing 7 to 14 mL per day. Fetal urine contains more urea, creatinine, and uric acid than plasma, as well as desquamated fetal cells, vernix, lanugo, and various secretions. Because these are hypotonic, the net effect is decreasing amionic fluid osmolality with advancing gestation. Pulmonary fluid contributes a small proportion of the amnionic volume, and fluid filtering through the placenta accounts for the rest.
The volume of amnionic fluid at each week of gestation is quite variable. In general, the volume increases by 10 mL per week at 8 weeks and increases up to 60 mL per week at 21 weeks, then declines gradually back to a steady state by 33 weeks (Brace and Wolf, 1989). The usual amnionic fluid volume thus increases from 50 mL at 12 weeks to 400 mL at midpregnancy and 1000 mL at term (Gillibrand, 1969).
Amnionic fluid serves to cushion the fetus, allowing musculoskeletal development and protecting it from trauma. It also maintains temperature and has a minimal nutritive function. Epidermal growth factor (EGF) and EGF6-like growth factors, such as transforming growth factor-a, are present in amnionic fluid. Ingestion of amnionic fluid into the lung and gastrointestinal tract may promote growth and differentiation of these tissues by inspiration and swallowing amnionic fluid. PTH-rP7 and endothelin-1 also are present in amnionic fluid, and it has been proposed that these peptides may be involved in fetal development. Both act as growth factors in selected cells, and PTH-rP promotes surfactant synthesis cells in type II pneumonocytes in vitro (Rubin and co-workers, 1994).
A more important function, however, is to promote the normal growth and development of the lungs and gastrointestinal tract. Animal studies have shown that pulmonary hypoplasia can be produced by draining off amnionic fluid, by banding the trachea to prevent "inhalation" of fluid into the lungs, by chronically draining pulmonary fluid through the trachea, and by physically preventing the prenatal chest excursions that mimic breathing (Adzick and associates, 1984; Alcorn and colleagues, 1977). Thus the formation of intrapulmonary fluid and, at least as important, the alternating egress and retention of fluid in the lungs by breathing movements, are essential to normal pulmonary development. Clinical implications of oligohydramnios and pulmonary hypoplasia are discussed in Chapter 31 (p. 822).
UMBILICAL CORD AND RELATED STRUCTURES
The yolk sac and the umbilical vesicle into which it develops are quite prominent early in pregnancy. At first, the embryo is a flattened disc interposed between amnion and yolk sac (Fig. 5-6). Because the dorsal surface grows faster than the ventral surface, in association with the elongation of the neural tube, the embryo bulges into the amnionic sac and the dorsal part of the yolk sac is incorporated into the body of the embryo to form the gut. The allantois projects into the base of the body stalk from the caudal wall of the yolk sac or, later, from the anterior wall of the hindgut.
As pregnancy advances, the yolk sac becomes smaller and its pedicle relatively longer. By about the middle of the third month, the expanding amnion obliterates the exocoelom, fuses with the chorion laeve, and covers the bulging placental disc and the lateral surface of the body stalk, which is then called the umbilical cord, or funis. Remnants of the exocoelom in the anterior portion of the cord may contain loops of intestine, which continue to develop outside the embryo. Although the loops are later withdrawn, the apex of the midgut loop retains its connection with the attenuated vitelline duct. The duct terminates in a crumpled, highly vascular sac 3 to 5 cm in diameter lying on the surface of the placenta between amnion and chorion or in the membranes just beyond the placental margin, where occasionally it may be identified at term.
The cord at term normally has two arteries and one vein. The right umbilical vein usually disappears early during fetal development, leaving only the original left vein. Sections of any portion of the cord frequently reveal, near the center, the small duct of the umbilical vesicle, lined by a single layer of flattened or cuboid epithelial cells. In sections just beyond the umbilicus, but never at the maternal end of the cord, another duct representing the allantoic remnant is occasionally found. The intra-abdominal portion of the duct of the umbilical vesicle, which extends from umbilicus to intestine, usually atrophies and disappears, but occasionally it remains patent, forming a Meckel diverticulum. The most common vascular anomaly is the absence of one umbilical artery.
STRUCTURE AND FUNCTION
The umbilical cord, or funis, extends from the fetal umbilicus to the fetal surface of the placenta or chorionic plate. Its exterior is dull white, moist, and covered by amnion, through which three umbilical vessels may be seen. Its diameter is 0.8 to 2.0 cm, with an average length of 55 cm and a range of 30 to 100 cm. Generally, cord length less than 30 cm is considered abnormally short (Benirschke and Kauffman, 2000). Folding and tortuosity of the vessels, which are longer than the cord itself, frequently create nodulations on the surface, or false knots, which are essentially varices. The extracellular matrix, which is a specialized connective tissue, consists of Wharton jelly (Figs. 5-18 and 5-19). After fixation, the umbilical vessels appear empty, but Figure 5-19 more accurately is representative of the situation in vivo, when the vessels are not emptied of blood. The two arteries are smaller in diameter than the vein. When fixed in their normally distended state, the umbilical arteries exhibit transverse intimal folds of Hoboken across part of their lumens (Chacko and Reynolds, 1954). The mesoderm of the cord, which is of allantoic origin, fuses with that of the amnion.
Blood flows from the umbilical vein by two routes—the ductus venosus, which empties directly into the inferior vena cava, and numerous smaller openings into the fetal hepatic circulation—and then into the inferior vena cava by the hepatic vein. The blood takes the path of least resistance through these alternate routes. Resistance in the ductus venosus is controlled by a sphincter situated at the origin of the ductus at the umbilical recess and innervated by a branch of the vagus nerve.
True umbilical knot
Anatomically, the umbilical cord can be regarded as a fetal membrane. The vessels contained in the cord are characterized by spiraling or twisting. The spiraling may occur in a clockwise (dextral) or anticlockwise (sinistral) direction. The anticlockwise spiral is present in 50 to 90 percent of cases. It is believed that the spiraling serves to attenuate "snarling," which occurs in all hollow cylinders subjected to torsion. Boyd and Hamilton (1970) note that these twists are not really spirals, but rather they are cylindrical helices in which a constant curva-ture is maintained equidistant from the central axis. Benirschke and Kauffman (2000) reported that 11 is the average number of helices in the cord.
FETAL GROWTH AND DEVELOPMENT
The high purposes of obstetrics are to maintain the health of the pregnant woman and to ensure the optimal well-being of the newborn. To this end, contemporary obstetrical research focuses on the physiology and pathophysiology of the fetus, its development, and its environment.
An important direct result of this research is that the status of the fetus has been elevated to that of a patient who, in large measure, can be given the same meticulous care that obstetricians provide for pregnant women. In the course of these studies it has become apparent that the conceptus is the dynamic force in the pregnancy unit. In general, the maternal organism responds passively to signals emanating from embronic-fetal and extraembryonic tissues. The contributions of the conceptus to implantation, maternal recognition of pregnancy, immunological acceptance, endocrine function, nutrition, and parturition are enormous, and absolutely essential for successful pregnancy .
DETERMINATION OF GESTATIONAL AGE
Several different terms are used to define the duration of pregnancy, and thus fetal age, but these are somewhat confusing. Gestational age or menstrual age is the time elapsed since the first day of the last menstrual period, a time that actually precedes conception. This starting time, which is usually about 2 weeks before ovulation and fertilization and nearly 3 weeks before implantation of the blastocyst, has traditionally been used because most women know when their last period was but not when they last ovulated, although the increasing use of infertility therapy has changed this somewhat. Embryologists, however, describe embryo-fetal development in days or weeks from the time of ovulation (ovulation age) or conception (postconceptional age), the latter two being nearly identical.
Obstetricians customarily calculate gestational age as menstrual age of a given pregnancy. About 280 days, or 40 weeks, elapse on average between the first day of the last menstrual period and the birth of the fetus; 280 days correspond to 9 1/3 calendar months, or 10 units of 28 days each. The unit of 28 days has been referred to, commonly but imprecisely, as a lunar month of pregnancy; actually, the time from one new moon to the next is 29 1/2 days. A quick estimate of the due date of a pregnancy based on menstrual cycle can be made as follows: add 7 days to the first day of the last menstrual period and subtract 3 months. For example, if the first day of the last menses was June 8, the due date of this pregnancy is 06 08 + 7 (days) minus 3 (months) = 03 15, or March 15 of the following year. As noted in Chapter 41 (p. 116), however, many women now undergo first or early second trimester ultrasound examination to confirm gestational age, and the sonographic estimate is usually a few days later than that determined by the last period. To rectify this inconsistency—and to reduce the number of pregnancies diagnosed as postterm—some authorities suggest assuming that the average pregnancy is actually 283 days long instead of 280, and thus adding 10 days to the last menses instead of 7 (Olsen and Clausen, 1998).
The period of gestation can also be divided into three units of three calendar months each, or three trimesters, because important obstetrical milestones can be designated conveniently by trimesters. The possibility of spontaneous abortion, for example, is limited principally to the first trimester, whereas the likelihood of survival of the infant born preterm is increased greatly in pregnancies that reach the third trimester.
OVUM, ZYGOTE, AND BLASTOCYST
During the first 2 weeks after ovulation, several successive phases of development can be identified:
2. Fertilization of the ovum.
3. Formation of free blastocyst.
4. Implantation of the blastocyst .
Primitive chorionic villi are formed soon after implantation. With the development of chorionic villi, it is conventional to refer to the products of conception not as a fertilized ovum, or zygote, but as an embryo. The early stages of preplacental development, and formation of the placenta, are described in Chapter 5.
The embryonic period commences at the beginning of the third week after ovulation/fertilization, which coincides in time with the expected day that the next menstruation would have started. Most pregnancy tests that measure human chorionic gonadotropin (hCG) use are positive by this time (Chap. 2, p. 26), and the embryonic disc is well defined. The body stalk is differentiated; the chorionic sac is approximately 1 cm in diameter (Figs. 7-2 and 7-3). There is a true intervillous space that contains maternal blood and villous cores in which angioblastic chorionic mesoderm can be distinguished.
By the end of the fourth week after ovulation, the chorionic sac is 2 to 3 cm in diameter, and the embryo is about 4 to 5 mm in length (Fig. 7-4). Partitioning of the primitive heart begins in the middle of the fourth week. Arm and leg buds are present, and the amnion is beginning to unsheathe the body stalk, which thereafter becomes the umbilical cord.
At the end of the sixth week after fertilization, the embryo is 22 to 24 mm in length, and the head is quite large compared with the trunk. The heart is completely formed. Fingers and toes are present, and the arms bend at the elbows. The upper lip is complete and the external ears form definitive elevations on either side of the head.
The end of the embryonic period and the beginning of the fetal period is arbitrarily designated by most embryologists to occur 8 weeks after fertilization, or 10 weeks after the onset of the last menstrual period. At this time, the embryo-fetus is nearly 4 cm long. The major portion of lung development is yet to occur, but few other new major body structures are formed after this time. Development during the fetal period of gestation consists of growth and maturation of structures that were formed during the embryonic period.
12 GESTATIONAL WEEKS. By the end of the 12th week of pregnancy, when the uterus usually is just palpable above the symphysis pubis, the crown-rump length of the fetus is 6 to 7 cm (Fig. 7-5). Centers of ossification have appeared in most of the fetal bones, and the fingers and toes have become differentiated. Skin and nails have developed and scattered rudiments of hair appear; the external genitalia are beginning to show definitive signs of male or female gender. The fetus begins to make spontaneous movements.
16 GESTATIONAL WEEKS. By the end of the 16th week, the crown-rump length of the fetus is 12 cm, and the weight is 110 g. Gender can be correctly determined by experienced observers by inspection of the external genitalia by 14 (menstrual) weeks.
20 GESTATIONAL WEEKS. The end of the 20th week is the midpoint of pregnancy as estimated from the beginning of the last normal menstrual period. The fetus now weighs somewhat more than 300 g, and the weight begins to increase in a linear manner. The fetal skin has become less transparent, a downy lanugo covers its entire body, and some scalp hair has developed.
24 GESTATIONAL WEEKS. By the end of the 24th week, the fetus weighs about 630 g. The skin is characteristically wrinkled, and fat deposition begins. The head is still comparatively quite large; eyebrows and eyelashes are usually recognizable. The canalicular period of lung development, during which the bronchi and bronchioles enlarge and alveolar ducts develop, is nearly completed. A fetus born at this period will attempt to breathe, but most will die because the terminal sacs, required for gas exchange, have not yet formed.
28 GESTATIONAL WEEKS. By the end of the 28th week, a crown-rump length of about 25 cm is attained and the fetus weighs about 1100 g. The thin skin is red and covered with vernix caseosa. The pupillary membrane has just disappeared from the eyes. An infant born at this time moves the limbs quite energetically and cries weakly. The otherwise normal infant of this age has a 90 percent chance of intact survival.
32 GESTATIONAL WEEKS. At the end of 32 gestational weeks, the fetus has attained a crown-rump length of about 28 cm and a weight of about 1800 g. The surface of the skin is still red and wrinkled. Barring other complications, infants born at this period usually survive intact.
36 GESTATIONAL WEEKS. At the end of 36 weeks gestation, the average crown-rump length of the fetus is about 32 cm and the weight is about 2500 g. Because of the deposition of subcutaneous fat, the body has become more rotund, and the previous wrinkled appearance of the face has been lost. Infants born at this time have an excellent chance of survival with proper care.
40 GESTATIONAL WEEKS. Term is reached at 40 weeks from the onset of the last menstrual period. At this time, the fetus is fully developed, with the characteristic features of the newborn infant to be described here. The average crown-rump length of the fetus at term is about 36 cm, and the weight is approximately 3400 g, with variations to be discussed subsequently.
LENGTH OF FETUS. Because of the variability in the length of the legs and the difficulty of maintaining them in extension, measurements corresponding to the sitting height (crown-to-rump) are more accurate than those corresponding to the standing height. The average sitting height and weight of the fetus at the end of each lunar month were determined by Streeter (1920) from 704 specimens. These values are similar to those found more recently and shown in Table 7-1. Such values are approximate, but generally, length is a more accurate criterion of gestational age than weight.
WEIGHT OF THE NEWBORN. The average term infant in the United States at birth weighs about 3000 to 3600 g, depending upon race, parental economic status, size of the parents, parity of the mother, and altitude, with boys about 100 g (3 oz) heavier than girls. During the second half of pregnancy, the fetal weight increases in a linear manner with time until about the 37th week of gestation, and then the rate slows variably. The principal determinants of fetal growth late in pregnancy are related, in large part, to factors influenced by the socioeconomic status of the mother, such as diet, smoking, or substance abuse. In general, the greater the socioeconomic deprivation, the slower the rate of fetal growth late in pregnancy.
Birthweights over 5000 g occur occasionally (Chap. 29, p. 757), but many tales of huge babies vastly exceeding this figure are based on hearsay or inaccurate measurements at best. Presumably, the largest baby recorded in the medical literature is that described by Belcher (1916), a stillborn female weighing 11,340 g (25 lb). Term infants, however, frequently weigh less than 3200 g, and sometimes as little as 2250 g (5 lb) or even less. It was customary in the past, when the birthweight was 2500 g or less, to classify the infant as preterm even though in some cases the low birthweight was not the consequence of preterm birth but rather the result of restricted growth.
MATERNAL ADAPTATIONS TO PREGNANCY
The anatomical, physiological, and biochemical adaptations to pregnancy are profound. Many of these changes begin soon after fertilization and continue throughout gestation, and most of these remarkable adaptations occur in response to physiological stimuli provided by the fetus. Equally astounding is that the woman who was pregnant is returned almost completely to her pre-pregnancy state after delivery and lactation. The understanding of these adaptations to pregnancy remains a major goal of obstetrics, and without such knowledge, it is almost impossible to understand the disease processes that can threaten women during pregnancy and the puerperium.
Because of these physiological adaptations, in some cases there are marked aberrations that would be perceived as abnormal in the nonpregnant state. For example, cardiovascular changes normally include substantive increases in blood volume and cardiac output, with hemodynamic adaptations that accompany them. This "high-output state" resembles thyrotoxicosis and other abnormal states. At the same time, underlying heart disease may lead to cardiac failure with these burdens.
Thus, physiological adaptations of normal pregnancy can be misinterpreted as disease, but they also may unmask or worsen preexisting disease. A number of laboratory values may appear abnormal, for example, pregnancy hypervolemia is accompanied by plasma volume expansion out of proportion to red cell mass increase. The result is so-called "physiological anemia" that is a major misnomer. The impact of these marked physiological changes on underlying disease, and vice versa, are considered in some detail in Section XII, which deals with medical and surgical complications of pregnancy.
In the nonpregnant woman, the uterus is an almost-solid structure weighing about 70 g and with a cavity of 10 mL or less During pregnancy, the uterus is transformed into a relatively thin-walled muscular organ of sufficient capacity to accommodate the fetus, placenta, and amnionic fluid The total volume of the contents at term averages about 5 L but may be 20 L or more, so that by the end of pregnancy the uterus has achieved a 500 to 1000 times greater capacity than in the nonpregnant state There is a corresponding increase in uterine weight, and at term, the organ weighs approximately 1100 g
During pregnancy, uterine enlargement involves stretching and marked hypertrophy of muscle cells, whereas the production of new myocytes is limited. The myometrial smooth muscle cells are surrounded by an irregular array of collagen fibrils. The force of contraction is transmitted from the contractile proteins of the myocytes to the surrounding connective tissue through the collagen reticulum.
Accompanying the increase in size of muscle cells is an accumulation of fibrous tissue, particularly in the external muscle layer, together with a considerable increase in elastic tissue. The network that is formed adds materially to the strength of the uterine wall. Concomitantly, there is a great increase in size and number of blood vessels and lymphatics. The veins that drain the placental site are transformed into large uterine sinuses, and there is hypertrophy of the nerves exemplified by the increase in size of the Frankenhauser cervical ganglion.
During the first few months, uterine hypertrophy is probably stimulated chiefly by the action of estrogen and perhaps that of progesterone. It is apparent that early hypertrophy is not entirely in response to mechanical distention by the products of conception, because similar uterine changes are observed with ectopic pregnancy But after about 12 weeks, the increase in uterine size is in large part related in some manner to the effect of pressure exerted by the expanding prducts of conception.
During the first few months of pregnancy, the uterine walls become considerably thicker, but as gestation advances the walls gradually thin. At term, the walls of the corpus are only about 1.5 cm or less in thickness. Early in pregnancy, the uterus loses the firmness and resistance characteristic of the nonpregnant organ. In the later months, the uterus is changed into a muscular sac with thin, soft, readily indentable walls, demonstrable by the ease with which the fetus usually can be palpated.
Uterine enlargement is not symmetrical, and it is most marked in the fundus. The differential growth is readily apparent by observing the relative positions of the attachments of the fallopian tubes and ovarian and round ligaments. In the early months of pregnancy, these structures attach only slightly below the apex of the fundus, whereas in the later months, they are located slightly above the middle of the uterus (see Fig. 3-9, p. 41). The position of the placenta also influences the extent of uterine hypertrophy, because the portion of the uterus surrounding the placental site enlarges more rapidly than does the rest.
ARRANGEMENT OF THE MUSCLE CELLS.
The uterine musculature during pregnancy is arranged in three strata:
1. An external hoodlike layer, which arches over the fundus and extends into the various ligaments.
2. An internal layer, consisting of sphincter-like fibers around the orifices of the tubes and the internal os.
3. Lying between these two, a dense network of muscle fibers perforated in all directions by blood vessels.
The main portion of the uterine wall is formed by the middle layer, which consists of an interlacing network of muscle fibers between which extend the blood vessels. Each cell in this layer has a double curve, so that the interlacing of any two gives approximately the form of the figure eight. As a result of this arrangement, when the cells contract after delivery they constrict the penetrating blood vessels and thus act as ligatures.
The muscle cells composing the uterine wall in pregnancy, especially in its lower portion, overlap one another like shingles on a roof. One end of each fiber arises beneath the serosa of the uterus and extends obliquely downward and inward toward the decidua, forming a large number of muscular lamellae that are interconnected by short muscular processes.
UTERINE SIZE, SHAPE, AND POSITION.
For the first few weeks the uterus maintains its original pear shape, but as pregnancy advances the corpus and fundus assume a more globular form, becoming almost spherical by 12 weeks. Subsequently, the organ increases more rapidly in length than in width and assumes an ovoid shape. By the end of 12 weeks, the uterus has become too large to remain totally within the pelvis. As the uterus continues to enlarge, it contacts the anterior abdominal wall, displaces the intestines laterally and superiorly, and continues to rise, ultimately reaching almost to the liver. As the uterus rises, tension is exerted upon the broad ligaments and upon the round ligaments.
With the pregnant woman standing, the longitudinal axis of the uterus corresponds to an extension of the axis of the pelvic inlet. The abdominal wall supports the uterus and, unless it is quite relaxed, maintains this relation between the long axis of the uterus and the axis of the pelvic inlet. When the pregnant woman is supine, the uterus falls back to rest upon the vertebral column and the adjacent great vessels, especially the inferior vena cava and the aorta.
With ascent of the uterus from the pelvis, it usually undergoes rotation to the right, and this dextrorotation likely is caused by the rectosigmoid on the left side of the pelvis.
From the first trimester onward, the uterus undergoes irregular contractions, which are normally painless. In the second trimester, these contractions may be detected by bimanual examination. Because attention was first called to this phenomenon in 1872 by J. Braxton Hicks, the contractions have been known by his name. Such contractions appear unpredictably and sporadically, are usually nonrhythmic, and their intensity varies between approximately 5 and 25 mm Hg (Alvarez and Caldeyro-Barcia, 1950). Until the last month of gestation, Braxton Hicks contractions are infrequent, but increase during the last week or two. At this time, the contractions may develop as often as every 10 to 20 minutes and may also assume some degree of rhythmicity. Late in pregnancy, these contractions may cause some discomfort and account for so-called false labor
During pregnancy, there is pronounced softening and cyanosis of the cervix, often demonstrable as early as a month after conception. The factors responsible for these changes are increased vascularity and edema of the entire cervix, together with hypertrophy and hyperplasia of the cervical glands. Although the cervix contains a small amount of smooth muscle, its major component is connective tissue. The cervix will undergo a rearrangement of its collagen-rich connective tissue, producing a 12-fold reduction in mechanical strength by term (Rechberger and colleagues, 1988).
The glands of the cervix undergo such marked proliferation that by the end of pregnancy they occupy approximately half of the entire cervical mass, rather than a small fraction as in the nonpregnant state. Moreover, the septa separating the glandular spaces become progressively thinner, resulting in the formation of a structure resembling a honeycomb, the meshes of which are filled with tenacious mucus. Soon after conception, a clot of very thick mucus obstructs the cervical canal. At the onset of labor, if not before, this so-called mucus plug is expelled, resulting in a bloody show. The glands near the external os proliferate beneath the stratified squamous epithelium of the portio vaginalis. These are customarily red and velvety in appearance and are covered by columnar epithelium. These normal pregnancy-induced changes represent an extension, or eversion, of the proliferating columnar endocervical glands. This tissue tends to be friable and bleeds even with minor trauma, such as with taking Pap smears.
There is a change in the consistency of the cervical mucus during pregnancy. In the great majority of pregnant women, cervical mucus, spread and dried on a glass slide, is characterized by fragmentary crystallization, or beading, typical of the effect of progesterone. In some women, arborization of the crystals, or ferning, is observed.
During pregnancy, basal cells near the squamocolumnar junction histologically are likely to be prominent in size, shape, and staining qualities. These changes are considered to be estrogen induced. The frequency of less-than-optimal Pap smears is increased in the pregnant woman (Kost and associates, 1993).
In the later months of pregnancy, reddish, slightly depressed streaks commonly develop in the skin of the abdomen and sometimes in the skin over the breasts and thighs in about half of pregnant women In multiparous women, in addition to the reddish striae of the present pregnancy, glistening, silvery lines that represent the cicatrices of previous striae frequently are seen
Occasionally the muscles of the abdominal walls do not withstand the tension to which they are subjected, and the rectus muscles separate in the midline, creating a diastasis recti of varying extent. If severe, a considerable portion of the anterior uterine wall is covered by only a layer of skin, attenuated fascia, and peritoneum.
In many women, the midline of the abdominal skin becomes markedly pigmented, assuming a brownish-black color to form the linea nigra Occasionally, irregular brownish patches of varying size appear on the face and neck, giving rise to chloasma or melasma gravidarum (mask of pregnancy) There is also accentuation of pigment of the areolae and genital skin Fortunately, this usually disappears, or at least regresses considerably after delivery (Chap 54, p 1430) Oral contraceptives may cause similar pigmentation in these same women There is very little known of the nature of these pigmentary changes, although melanocyte-stimulating hormone, a polypeptide similar to corticotropin, has been shown to be elevated remarkably from the end of the second month of pregnancy until term (see also Chap 6) Estrogen and progesterone are reported to have some melanocyte-stimulating effects Vaughn Jones and Black (1999) attribute most changes to estrogen
In response to the rapidly growing fetus and placenta and their increasing demands, the pregnant woman undergoes metabolic changes that are numerous and intense. Certainly no other physiological event in postnatal life induces such profound metabolic alterations.
Most of the increase in weight during pregnancy is attributable to the uterus and its contents, the breasts, and increases in blood volume and extravascular extracellular fluid A smaller fraction of the increased weight is the result of metabolic alterations that result in an increase in cellular water and deposition of new fat and protein, so-called maternal reserves Hytten (1991) reported an average weight gain of 125 kg
Increased water retention is a normal physiological alteration of pregnancy. This is mediated, at least in part, by a fall in plasma osmolality of approximately 10 mOsm/kg induced by a resetting of osmotic thresholds for thirst and vasopressin secretion (Lindheimer and Davidson, 1995). As shown in Figure 8-3, this phenomenon is functioning by early pregnancy.
At term, the water content of the fetus, placenta, and amnionic fluid amounts to about 3.5 L. Another 3.0 L accumulates as a result of increases in the maternal blood volume and in the size of the uterus and the breasts. Thus, the minimum amount of extra water that the average women retains during normal pregnancy is about 6.5 L. Clearly demonstrable pitting edema of the ankles and legs is seen in a substantial proportion of pregnant women, especially at the end of the day. This accumulation of fluid, which may amount to a liter or so, is caused by an increase in venous pressure below the level of the uterus as a consequence of partial occlusion of the vena cava. A decrease in interstitial colloid osmotic pressure induced by normal pregnancy also favors edema late in pregnancy (Oian and co-workers, 1985).
Longitudinal studies of body composition have shown a progressive increase in total body water and fat mass during pregnancy. It has been known for decades that both initial maternal weight and the weight gained during pregnancy are highly associated with birthweight. It is unclear, however, what role maternal fat or water have in fetal growth. Recent studies in well-nourished term women suggest that maternal body water, rather than fat, contributes more significantly to infant birthweight (Lederman and associates, 1999; Mardones-Santander and associates, 1998).
The products of conception, as well as uterus and maternal blood, are relatively rich in protein rather than fat or carbohydrate. At term, the fetus and placenta together weigh about 4 kg and contain approximately 500 g of protein, or about half of the total pregnancy increase (Hytten and Leitch, 1971). The remaining 500 g is added to the uterus as contractile protein, to the breasts primarily in the glands, and to the maternal blood as hemoglobin and plasma proteins.
From nitrogen balance studies in pregnant women, it appears that actual nitrogen use is only 25 percent (Calloway, 1974). Therefore, daily requirements for protein intake during pregnancy are increased appreciably to correct for this. Equally important is the ingestion of adequate carbohydrates and fat. If these are not consumed in adequate amounts, energy requirements must be met by catabolism of maternal protein stores. Amino acids used for energy are not available for synthesis of maternal protein. With increasing intake of fat and carbohydrates as energy sources, less dietary protein is required to maintain positive nitrogen balance.
Normal pregnancy is characterized by mild fasting hypoglycemia, postprandial hyperglycemia, and hyperinsulinemia (Fig. 8-4). The fasting plasma glucose concentration falls somewhat, possibly due to increased plasma levels of insulin. This cannot be explained by a change in the metabolism of insulin because its half-life during pregnancy is not changed (Lind and associates, 1977).
The increased basal level of plasma insulin observed in normal pregnancy is associated with several unique responses to glucose ingestion. For example, after an oral glucose meal, there is both prolonged hyperglycemia and hyperinsulinemia in pregnant women, with a greater suppression of glucagon (Phelps and associates, 1981). The purpose of such a mechanism is likely to ensure a sustained or maintained postprandial supply of glucose to the fetus. This response is consistent with a pregnancy-induced state of peripheral resistance to insulin, which is suggested by three observations:
1. Increased insulin response to glucose.
2. Reduced peripheral uptake of glucose.
3. Suppressed glucagon response.
The mechanism(s) responsible for insulin resistance is not completely understood. Progesterone and estrogen may act, directly or indirectly, to mediate this resistance. Plasma levels of placental lactogen increase with gestation, and this protein hormone is characterized by growth hormone-like action that may result in increased lipolysis with liberation of free fatty acids (Freinkel, 1980). The increased concentration of circulating free fatty acids also may facilitate increased tissue resistance to insulin.
The mechanisms cited ensure that a continuous supply of glucose is available for transfer to the fetus. The pregnant woman, however, changes rapidly from a postprandial state characterized by elevated and sustained glucose levels to a fasting state characterized by decreased plasma glucose and amino acids such as alanine. There also are higher plasma concentrations of free fatty acids, triglycerides, and cholesterol in the pregnant woman during fasting (Fig. 8-5). Freinkel and colleagues (1985) have referred to this pregnancy-induced switch in fuels from glucose to lipids as accelerated starvation. Certainly, when fasting is prolonged in the pregnant woman, these alterations are exaggerated and ketonemia rapidly appears.
Hornnes and Kuhl (1980) measured glucagon and insulin responses to a standard glucose stimulus late in normal pregnancy and again in the same women postpartum. The peak insulin response to glucose infusion was increased fourfold in late pregnancy. In contrast, plasma glucagon concentrations were suppressed, and the degree was similar in late pregnancy and the puerperium. These results are consistent with the view that ß-cell sensitivity to a glucose challenge is increased significantly in normal pregnant women, but that the a-cell sensitivity to a glucose stimulus is unaltered.
The concentrations of lipids, lipoproteins, and apolipoproteins in plasma increase appreciably during pregnancy. Desoye and co-workers (1987) found that there were positive correlations between the concentrations of lipids shown in Figure 8-5 and those of estradiol, progesterone, and placental lactogen.
Plasma lipoprotein cholesterol levels also increase significantly. Low-density lipoprotein cholesterol (LDL-C) levels peak at approximately week 36, likely the consequence of the hepatic effects of estradiol and progesterone (Desoye and associates, 1987). High-density lipoprotein cholesterol (HDL-C) peaks at week 25, decreases until week 32, and remains constant for the remainder of pregnancy. The initial increase is believed to be caused by estrogen. High-density lipoprotein-2 and -3 cholesterol levels peak at approximately 28 weeks and remain unchanged throughout the remainder of pregnancy. Brizzi and colleagues (1999) have suggested that changes in the low-density lipoprotein (LDL) pattern during normal pregnancy might be used to identify those women who later in life may be predisposed to atherogenesis.
After delivery, the concentrations of these lipids, lipoproteins, and apolipoproteins decrease at different rates (Desoye and co-workers, 1987). Lactation increases the rate of decrease of many of these compounds (Darmady and Postle, 1982).
Hytten and Thomson (1968) and Pipe and co-workers (1979) concluded that storage of fat occurs primarily during midpregnancy. This fat is deposited mostly in central rather than peripheral sites. Later in pregnancy, as fetal nutritional demands increase remarkably, maternal fat storage decreases. Hytten and Thomson (1968) cited some evidence that progesterone may act to reset a lipostat in the hypothalamus, and at the end of pregnancy the lipostat returns to its previous nonpregnant level and the added fat is lost. Such a mechanism for energy storage, theoretically at least, might protect the mother and fetus during times of prolonged starvation or hard physical exertion.
The requirements for iron during pregnancy are considerable and often exceed the amounts available (p. 178). With respect to most other minerals, pregnancy induces little change in their metabolism other than their retention in amounts equivalent to those used for growth of fetal and, to a lesser extent, maternal tissues (Chaps. 7, p. 139 and 10, p. 235).
During pregnancy, calcium and magnesium plasma levels decline, the reduction probably reflecting for the most part the lowered plasma protein concentration and, in turn, the consequent decrease in the amount bound to protein. Bardicef and colleagues (1995), however, concluded that pregnancy is a state of magnesium depletion. They showed that total and ionized magnesium levels were significantly lower in normal pregnancy compared with nonpregnant women. Fogh-Andersen and Schultz-Larsen (1981) demonstrated a small but significant increase in free calcium ion concentration in late pregnancy by correcting for blood pH changes. Serum phosphate levels are within the nonpregnant range. The renal threshold for inorganic phosphate excretion is elevated in pregnancy due to increased calcitonin (Weiss and colleagues, 1998). Cole and co-workers (1987) reported that bone turnover was reduced during early pregnancy, returned toward normal during the third trimester, and increased in postpartum lactating women.
As discussed subsequently (p 186), minute ventilation increases during pregnancy and this causes a respiratory alkalosis by lowering the PCO2 of blood A moderate reduction in plasma bicarbonate from 26 to about 22 mmol/L partially compensates for this As a result, there is only a minimal increase in blood pH This increase shifts the oxygen dissociation curve to the left and increases the affinity of maternal hemoglobin for oxygen (Bohr effect), thereby decreasing the oxygen-releasing capacity of maternal blood Thus, the hyperventilation that results in a reduced maternal PCO2 facilitates transport of carbon dioxide from the fetus to the mother but appears to impair release of oxygen from maternal blood to the fetus The increase in blood pH, however, although minimal, stimulates an increase in 2, 3-diphosphoglycerate in maternal erythrocytes (Tsai and deLeeuw, 1982) This counteracts the Bohr effect by shifting the oxygen dissociation curve back to the right, facilitating oxygen release to the fetus
Despite large accumulations during pregnancy of sodium and potassium, the serum concentration of these electrolytes decreases During normal pregnancy, nearly 1000 mEq of sodium and 300 mEq of potassium are retained (Lindheimer and colleagues, 1987) Despite that their glomerular filtration is increased, sodium and potassium excretion are unchanged during pregnancy (Brown and colleagues, 1986, 1988) Thus, their fractional excretion is decreased, and it has been postulated that progesterone counteracts the natriuretic and kaliuretic effects of aldosterone
The maternal blood volume increases markedly during pregnancy In studies of normal women, the blood volumes at or very near term averaged about 40 to 45 percent above their nonpregnant levels (Pritchard, 1965; Whittaker and associates, 1996) The degree of expansion varies considerably, in some women there is only a modest increase, while in others the blood volume nearly doubles A fetus is not essential for the development of hypervolemia during pregnancy, for increases in blood volume have been demonstrated in some women with hydatidiform mole (Pritchard, 1965)
Pregnancy-induced hypervolemia has several important functions:
1. To meet the demands of the enlarged uterus with its greatly hypertrophied vascular system.
2. To protect the mother, and in turn the fetus, against the deleterious effects of impaired venous return in the supine and erect positions.
3. To safeguard the mother against the adverse effects of blood loss associated with parturition.
Maternal blood volume starts to increase during the first trimester, expands most rapidly during the second trimester, and then rises at a much slower rate during the third trimester to plateau during the last several weeks of pregnancy.
Increased blood volume results from an increase in both plasma and erythrocytes. Although more plasma than erythrocytes is usually added to the maternal circulation, the increase in volume of erythrocytes is considerable, averaging about 450 mL, or an increase of about 33 percent (Pritchard and Adams, 1960). The importance of this increase in creating a demand for iron is discussed on page 178.
Moderate erythroid hyperplasia is present in the bone marrow, and the reticulocyte count is elevated slightly during normal pregnancy. This is almost certainly related to the increase in maternal plasma erythropoietin levels (Chap. 49, p. 1310). These levels increase after 20 weeks, corresponding to when erythrocyte production is most marked (Harstad and co-workers, 1992).
ATRIAL NATRIURETIC PEPTIDES. This group of biologically active peptides is synthesized and secreted by atrial myocytes. Three separate forms (a, ß, ?) have been isolated (Kangawa and co-workers, 1985). Atrial natriuretic peptide produces significant natriuresis and diuresis. It increases renal blood flow and glomerular filtration rate and decreases renin secretion. The actual mechanism(s) responsible for the natriuresis remains unclear, with evidence consistent for both a hemodynamically induced natriuresis and an inhibitory effect upon tubular sodium reabsorption (Wakitani and colleagues, 1985). Atrial natriuretic peptides also have been shown to reduce basal release of aldosterone from zona glomerulosa cells and to blunt corticotropin and angiotensin II-stimulated release of aldosterone as well (Atarashi and associates, 1984). Renin secretion is also inhibited by this peptide. Finally, atrial natriuretic peptides have a direct vasorelaxant action upon vascular smooth muscle stimulated by angiotesin II or norepinephrine.
Castro and associates (1994) summarized several studies done to evaluate plasma levels of atrial natriuretic peptide in normal and hypertensive pregnancy. The mean level rose by 40 percent over nonpregnant values by the third trimester and by 150 percent during the first week postpartum. These investigators hypothesized that atrial stretch receptors sense the expanded blood volume of pregnancy as normal to moderately increased. The marked rise in peptide levels during the first week postpartum is consistent with known hemodynamic changes and suggests that the hormone is involved in postpartum diuresis.
By comparison, Thomsen and colleagues (1994) investigated 10 healthy primigravid twin pregnancies and reported that all atrial natriuretic peptide levels during pregnancy were lower than values at 12 weeks postpartum. At 20, 28, and 32 weeks, plasma peptide levels were lower in twin than in singleton pregnancies. These observations may serve to explain in part the relatively increased plasma volume characteristic of women with twins compared with those with singleton fetuses.
HEMOGLOBIN CONCENTRATION AND HEMATOCRIT.
In spite of augmented erythropoiesis, hemoglobin concentration and the hematocrit decrease slightly during normal pregnancy. As a result, whole blood viscosity decreases (Huisman and colleagues, 1987). Hemoglobin concentration at term averages 12.5 m/dL and in 6 percent of women it is below 11.0 g/dL (see Fig. 49-1 and Table 49-1). Thus, in most women, a hemoglobin concentration below 11.0 g/dL, especially late in pregnancy, should be considered abnormal and usually due to iron deficiency rather than to hypervolemia of pregnancy.
IRON METABOLISM IRON STORES. Although the total body iron content averages about 4 g in men, in healthy young women of average size, it is probably half that amount. Commonly, iron stores of normal young women are only about 300 mg (Pritchard and Mason, 1964). As in men, heme iron in myoglobin and enzymes and transferrin-bound circulating iron together total only a few hundred milligrams. The total iron content of normal adult women ranges from 2.0 to 2.5 g.
IRON REQUIREMENTS. The iron requirements of normal pregnancy total about 1000 mg. About 300 mg are actively transferred to the fetus and placenta and about 200 mg are lost through various normal routes of excretion. These are obligatory losses and occur even when the mother is iron deficient. The average increase in the total volume of circulating erythrocytes of about 450 mL during pregnancy, when iron is available, uses another 500 mg of iron, because 1 mL of normal erythrocytes contains 1.1 mg of iron. Practically all of the iron for these purposes is used during the latter half of pregnancy. Therefore, the iron requirement becomes quite large during the second half of pregnancy, averaging 6 to 7 mg/day (Pritchard and Scott, 1970). Because this amount is not available from body stores in most women, the desired increase in maternal erythrocyte volume and hemoglobin mass will not develop unless exogenous iron is made available in adequate amounts. In the absence of supplemental iron, the hemoglobin concentration and hematocrit fall appreciably as the maternal blood volume increases. Hemoglobin production in the fetus, however, will not be impaired, because the placenta obtains iron from the mother in amounts sufficient for the fetus to establish normal hemoglobin levels even when the mother has severe iron-deficiency anemia.
The amount of iron absorbed from diet, together with that mobilized from stores, is usually insufficient to meet the demands imposed by pregnancy. This is true even though gastrointestinal tract iron absorption appears to be moderately increased during pregnancy (Hahn and associates, 1951). If the pregnant woman who is not anemic is not given supplemental iron, serum iron and ferritin concentrations decline during the second half of pregnancy (Fig. 8-7). The somewhat unexpected early pregnancy increases in serum iron and ferritin are thought to be due to minimal iron demands during the first trimester as well as to a positive iron balance because of amenorrhea. These values, summarized in Table 8-2, show that standard deviations for a given mean value are quite large. For example, in pregnant women with overt anemia and not given supplemental iron, serum ferritin levels can vary from 7 to 22 ng/mL during the third trimester. Also shown in Figure 8-7 is the increase in iron-binding capacity (transferrin) that occurs even when iron deficiency has been eliminated by oral iron supplementation.
BLOOD LOSS. Not all the iron added to the maternal circulation in the form of hemoglobin is necessarily lost from the mother. During normal vaginal delivery and through the next few days, only about half of the erythrocytes added to the maternal circulation during pregnancy are lost from the majority of women. These losses are by way of the placental implantation site, the placenta itself, the episiotomy or lacerations, and in the lochia. On the average, an amount of maternal erythrocytes corresponding to about 500 to 600 mL of predelivery blood is lost during and after vaginal delivery of a single fetus (Pritchard, 1965; Ueland, 1976). The average blood loss associated with cesarean delivery or with the vaginal delivery of twins is about 1000 mL, or nearly twice that lost with the delivery of a single fetus.
IMMUNOLOGICAL AND LEUKOCYTE FUNCTIONS
Pregnancy has been assumed to be associated with suppression of a variety of humoral and cellularly mediated immunological functions in order to accommodate the "foreign" semiallogeneic fetal graft (Chap 2, p 20) In fact, humoral antibody titers against several viruses—for example, herpes simplex, measles, and influenza A—decrease during pregnancy The decrease in titers, however, is accounted for by the hemodilutional effect of pregnancy The prevalence of a variety of autoantibodies is unchanged (Patton and colleagues, 1987) Furthermore, a-interferon, which is present in almost all fetal tissues and fluids, is most often absent in normally pregnant women (Chard and co-workers, 1986) There is evidence, as yet unexplained, that polymorphonuclear leukocyte chemotaxis and adherence functions are depressed beginning in the second trimester and continuing throughout pregnancy (Krause and associates, 1987) It is possible that these depressed leukocyte functions of pregnant women account in part for the improvement observed in some with autoimmune diseases and the possibly increased susceptibility to certain infections Thus, both function and absolute numbers of leukocytes appear to be important factors when considering the leukocytosis of normal pregnancy
The leukocyte count varies considerably during normal pregnancy. Usually it ranges from 5000 to 12,000/uL. During labor and the early puerperium it may become markedly elevated, attaining levels of 25,000 or even more; however, the concentration averages 14,000 to 16,000/uL (Taylor and co-workers, 1981). The cause for the marked increase is not known, but the same response occurs during and after strenuous exercise. It probably represents the reappearance in the circulation of leukocytes previously shunted out of the active circulation. During pregnancy there is a neutrophilia that consists predominantly of mature forms; however, an occasional myelocyte is found.
During pregnancy and the puerperium there are remarkable changes involving the heart and the circulation. The most important changes in cardiac function occur in the first eight weeks of pregnancy (McLaughlin and Roberts, 1999). Cardiac output is increased as early as the fifth week of pregnancy and this initial increase is a function of reduced systemic vascular resistance and an increase in heart rate. Between weeks 10 and 20, notable increases in plasma volume occur such that preload is increased. Ventricular performance during pregnancy is influenced by both the decrease in systemic vascular resistance and changes in pulsatile arterial flow. Vascular capacity increases, in part, due to an increase in vascular compliance. As discussed in the following section, multiple factors contribute to these changes in overall hemodynamic function, allowing the cardiovascular system to adjust to the physiological demands of the fetus while maintaining maternal cardiovascular integrity. These changes during the last half of pregnancy are graphically summarized in Figure 8-8, which also shows the important effects of maternal posture on hemodynamic events during pregnancy.
The resting pulse rate increases about 10 bpm during pregnancy (Stein and co-workers, 1999) As the diaphragm becomes progressively elevated, the heart is displaced to the left and upward, while at the same time it is rotated somewhat on its long axis As a result, the apex of the heart is moved somewhat laterally from its position in the normal nonpregnant state, and an increase in the size of the cardiac silhouette is found in radiographs (Fig 8-9) The extent of these changes is influenced by the size and position of the uterus, toughness of the abdominal muscles, and configurations of the abdomen and thorax Furthermore, normally pregnant women have some degree of benign pericardial effusion which may increase the cardiac silhouette (Enein and colleagues, 1987) Variability of these factors makes it difficult to identify precisely moderate degrees of cardiomegaly by simple x-ray studies
Katz and co-workers (1978) studied left ventricular performance during pregnancy and the puerperium using echocardiography. Both left ventricular wall mass and end-diastolic dimensions increased during pregnancy, as did heart rate, calculated stroke volume, and cardiac output. The changes in stroke volume were directly proportional to end-diastolic volume, implying, at least, that there is little change in the inotropic state of the myocardium during normal pregnancy. Sadaniantz and associates (1996) reported that these changes are not cumulative in subsequent pregnancies. In multifetal pregnancies, however, cardiac output is increased predominantly by increased inotropic effect (Veille and co-workers, 1985). The increased heart rate and inotropic contractility imply that cardiovascular reserve is reduced.
During pregnancy, some of the cardiac sounds may be altered. Cutforth and MacDonald (1966) obtained phonocardiograms at varying stages of pregnancy in 50 normal women and documented the following changes:
1. An exaggerated splitting of the first heart sound with increased loudness of both components; no definite changes in the aortic and pulmonary elements of the second sound; and a loud, easily heard third sound.
2. A systolic murmur in 90 percent of pregnant women, intensified during inspiration in some or expiration in others, and disappearing very shortly after delivery; a soft diastolic murmur transiently in 20 percent; and continuous murmurs arising from the breast vasculature in 10 percent.
Normal pregnancy induces no characteristic changes in the electrocardiogram, other than slight deviation of the electrical axis to the left as a result of the altered position of the heart.
CARDIAC OUTPUT. During normal pregnancy, arterial blood pressure and vascular resistance decrease while blood volume, maternal weight, and basal metabolic rate increase. Each of these events would be expected to affect cardiac output. It is now evident that cardiac output at rest, when measured in the lateral recumbent position, increases significantly beginning in early pregnancy (Duvekot and colleagues, 1993; Mabie and co-workers, 1994). It continues to increase and remains elevated during the remainder of pregnancy (Fig. 8-10). Typically, cardiac output in late pregnancy is appreciably higher when the woman is in the lateral recumbent position than when she is supine, because in the supine position the large uterus often impedes cardiac venous return. Ueland and Hansen (1969), for example, found cardiac output to increase 1100 mL (20 percent) when the pregnant woman was moved from her back onto her side. When she assumes the standing position after sitting, cardiac output in the pregnant woman falls to the same degree as in the nonpregnant woman (Easterling and associates, 1988).
During the first stage of labor, cardiac output increases moderately, and during the second stage, with vigorous expulsive efforts, it is appreciably greater (Fig. 8-10). After the substantively augmented cardiac output in the immediate puerperium, most of the pregnancy-induced increase is lost very soon after delivery.
HEMODYNAMIC VALUES FOR LATE PREGNANCY.
Clark and colleagues (1989) conducted studies of maternal cardiovascular hemodynamics that serve to define normal values late in pregnancy (Table 8-3). Right heart catheterization was performed in 10 healthy nulliparous women at 35 to 38 weeks, and again at 11 to 13 weeks postpartum. Late pregnancy was associated with the expected increases in heart rate, stroke volume, and cardiac output. Systemic vascular and pulmonary vascular resistance both decreased significantly, as did colloid osmotic pressure. Pulmonary capillary wedge pressure and central venous pressure did not change appreciably between late pregnancy and the puerperium. These investigators concluded that normal late pregnancy is not associated with hyperdynamic left ventricular function as determined by Starling function curves
The diaphragm rises about 4 cm during pregnancy (see Fig. 8-9). The subcostal angle widens appreciably as the transverse diameter of the thoracic cage increases about 2 cm. The thoracic circumference increases about 6 cm, but not sufficiently to prevent a reduction in the residual volume of air in the lungs created by the elevated diaphragm. Diaphragmatic excursion is actually greater during pregnancy than when nonpregnant.
A remarkable number of changes are observed in the urinary system as a result of pregnancy (Table 8-5) Kidney size increases slightly during pregnancy Bailey and Rolleston (1971), for example, found that the kidney was 15 cm longer during the early puerperium than when measured 6 months later The glomerular filtration rate (GFR) and renal plasma flow (RPF) increase early in pregnancy, the former as much as 50 percent by the beginning of the second trimester, and the latter not quite so much (Chesley, 1963; Dunlop, 1981) As shown in Figure 8-13, elevated glomerular filtration has been found by most investigators to persist to term, and this is despite that renal plasma flow decrease during late pregnancy Kallikrein, a tissue protease synthesized in cells of the distal renal tubule, is increased in several conditions associated with increased glomular perfusion in nonpregnant individuals Platts and colleagues (2000) studied urinary kallikrein excretion rates during human pregnancy and found increased excretion at 18 and 34 weeks which returned to nonpregnant levels at term The significance of these normal fluctuations in renal kallikrein excretion rates during pregnancy remains unknown
Most studies of renal function conducted during pregnancy have been performed while the subjects were supine, a position that late in pregnancy may produce marked systemic hemodynamic changes that lead to alterations in several aspects of renal function. Late in pregnancy, for instance, urinary flow and sodium excretion average less than half the excretion rate in the supine position compared with the lateral recumbent position.
Although posture clearly affects sodium and water excretion in late pregnancy, its impact on glomerular filtration and renal plasma flow is much more variable. For example, Chesley and Sloan (1964) found both to be reduced when the pregnant woman was in the supine position, whereas Dunlop (1976) identified inconsequential reduction. Pritchard (1955) detected decreases while supine compared with lateral recumbency in some, but not most of women studied in late pregnancy. Ezimokhai and associates (1981) reported that the late pregnancy decrease in renal plasma flow is not due simply to a positional effect.
LOSS OF NUTRIENTS. One unusual feature of the pregnancy-induced changes in renal excretion is the remarkably increased amounts of various nutrients in the urine. Amino acids and water-soluble vitamins are lost in the urine of pregnant women in much greater amounts than in nonpregnant women (Hytten and Leitch, 1971).
TESTS OF RENAL FUNCTION. During pregnancy the plasma concentrations of creatinine and urea normally decrease as a consequence of their increased glomerular filtration. At times, the urea concentration may be so low as to suggest impaired hepatic synthesis, which sometimes occurs with severe liver disease.
Creatinine clearance is a useful test to estimate renal function in pregnancy provided that complete urine collection is made over an accurately timed period. Urine concentration tests may give results that are misleading (Davison and colleagues, 1981). During the day, pregnant women tend to accumulate water in the form of dependent edema; and at night, while recumbent, they mobilize this fluid and excrete it via the kidneys. This reversal of the usual nonpregnant diurnal pattern of urinary flow causes nocturia, and the urine is more dilute than in the nonpregnant state. Failure of a pregnant woman to excrete concentrated urine after withholding fluids for approximately 18 hours does not signify renal damage. In fact, the kidney in these circumstances functions perfectly normally by excreting mobilized extracellular fluid of relatively low osmolality.
URINALYSIS. Glucosuria during pregnancy is not necessarily abnormal. The appreciable increase in glomerular filtration, together with impaired tubular reabsorptive capacity for filtered glucose, accounts in most cases for the glucosuria (Davison and Hytten, 1975). Chesley (1963) calculated that for these reasons alone about one sixth of all pregnant women should spill glucose in the urine. Even though glucosuria is common during pregnancy, the possibility of diabetes mellitus should not be ignored when it is identified.
Proteinuria is normally not evident during pregnancy except occasionally in slight amounts during or soon after vigorous labor. Higby and associates (1994) measured protein excretion in 270 normal women throughout pregnancy. Their mean 24-hour excretion was 115 mg, and the upper 95 percent confidence limit was 260 mg/day. There were no significant differences by trimester (Fig. 8-14). They also showed that albumin excretion is minimal and ranges from 5 to 30 mg/day. Lopez-Espinoza and colleagues (1986) measured serial albumin excretion using a sensitive radioimmunoassay in 14 healthy pregnant women. There was a slight rise from a median of 7 to 18 mg/24 hours from early to late pregnancy, however, albuminuria was not detected using conventional testing methods.
Hematuria, if not the result of contamination during collection, is compatible with a diagnosis of urinary tract disease (Chap. 47, p. 1259). Difficult labor and delivery, of course, can cause hematuria because of trauma to the lower urinary tract.
HYDRONEPHROSIS AND HYDROURETER. After the uterus rises completely out of the pelvis, it rests upon the ureters, compressing them at the pelvic brim. Increased intraureteral tonus above this level compared with that of the pelvic portion of the ureter has been identified (Rubi and Sala, 1968). Schulman and Herlinger (1975) found ureteral dilatation to be greater on the right side in 86 percent of pregnant women studied (Fig. 8-15). The unequal degrees of dilatation may result from a cushioning provided the left ureter by the sigmoid colon and perhaps from greater compression of the right ureter as the consequence of dextrorotation of the uterus. The right ovarian vein complex, which is remarkably dilated during pregnancy, lies obliquely over the right ureter and may contribute significantly to right ureteral dilatation.
Another possible mechanism causing hydronephrosis and hydroureter is from an effect of progesterone. Major support for this concept was provided by Van Wagenen and Jenkins (1939), who described further ureteral dilatation after removal of the monkey fetus but with the placenta left in situ. The relatively abrupt onset of dilatation in women at midpregnancy is more consistent with ureteral compression from an enlarging uterus rather than a hormonal effect.
Elongation accompanies distention of the ureter, which is frequently thrown into curves of varying size, the smaller of which may be sharply angulated. These so-called kinks are poorly named, because the term connotes obstruction. They are usually single or double curves, which when viewed in the radiograph taken in the same plane as the curve, appear as more or less acute angulations of the ureter (Fig. 8-15). Another exposure at right angles nearly always identifies them to be more gentle curves rather than kinks. The ureter undergoes not only elongation but frequently lateral displacement by the pressure of the enlarged uterus.
Thorp and colleagues (1999) studied 123 pregnant women throughout pregnancy and the puerperium and found that pregnancy was associated with an increase in urinary incontinence When women were asked to compare urinary tract function week by week throughout their pregnancies, they reported a steady deterioration in perceived bladder function Indeed, objective measures of urinary frequency and total daily urinary output increased throughout pregnancy
There are few significant anatomical changes in the bladder before 12 weeks. From that time onward, however, the increased size of the uterus, together with the hyperemia that affects all pelvic organs, and the hyperplasia of the muscle and connective tissues, elevates the bladder trigone and causes thickening of its posterior, or intraureteric, margin. Continuation of this process to the end of pregnancy produces marked deepening and widening of the trigone. The bladder mucosa undergoes no change other than an increase in the size and tortuosity of its blood vessels.
Using urethrocystometry, Iosif and colleagues (1980) found that bladder pressure in primigravidas increased from 8 cm H2O early in pregnancy to 20 cm H2O at term. To compensate for reduced bladder capacity, absolute and functional urethral lengths increased by 6.7 and 4.8 mm, respectively. Finally, to preserve continence, maximal intraurethral pressure increased from 70 to 93 cm H2O. Still, the majority of women will experience their initial episode of urinary incontinence during pregnancy. Indeed, loss of urine is always high in the differential diagnosis of the woman presenting with a question of ruptured membranes.
Toward the end of pregnancy, particularly in nulliparas in whom the presenting part often engages before labor, the entire base of the bladder is pushed forward and upward, converting the normal convex surface into a concavity. As a result, difficulties in diagnostic and therapeutic procedures are greatly increased. In addition, the pressure of the presenting part impairs the drainage of blood and lymph from the base of the bladder, often rendering the area edematous, easily traumatized, and probably more susceptible to infection. Both urethral pressure and length have been shown to be decreased in women following vaginal but not abdominal delivery (Van Geelen and co-workers, 1982). These investigators suggest that a weakness of the urethral sphincter mechanism due to pregnancy or delivery may play a role in the pathogenesis of urinary stress incontinence.
Normally there is little residual urine in nulliparas, but occasionally it develops in the multipara with relaxed vaginal walls and a cystocele. Incompetence of the ureterovesical valve may supervene, with the consequent probability of vesicoureteral reflux of urine.
As pregnancy progresses, the stomach and intestines are displaced by the enlarging uterus. As the result of the positional changes in these viscera, the physical findings in certain diseases are altered. The appendix, for instance, is usually displaced upward and somewhat laterally as the uterus enlarges, and at times it may reach the right flank.
Gastric emptying and intestinal transit times are delayed in pregnancy because of hormonal or mechanical factors. For example, this may be the result of progeste-rone or decreased levels of motilin, a hormonal peptide known to have smooth-muscle stimulating effects (Christofides and associates, 1982). Macfie and colleagues (1991) studied gastric emptying times using acetaminophen absorption and found these to be unchanged during each trimester and compared with nonpregnant women. During labor, however, and especially after administration of analgesic agents, gastric-emptying time is typically prolonged appreciably. A major danger of general anesthesia for delivery is regurgitation and aspiration of either food-laden or highly acidic gastric contents (Chap. 15, p. 366).
Pyrosis (heartburn) is common during pregnancy and is most likely caused by reflux of acidic secretions into the lower esophagus (Chap. 48, p. 1276). The altered position of the stomach probably contributes to its frequent occurrence; however, lower esophageal sphincter tone is also decreased. Intraesophageal pressures are lower and intragastric pressures higher in pregnant women. At the same time, esophageal peristalsis has lower wave speed and lower amplitude (Ulmsten and Sundstrom, 1978).
The gums may become hyperemic and softened during pregnancy and may bleed when mildly traumatized, as with a toothbrush. A focal, highly vascular swelling of the gums, the so-called epulis of pregnancy, develops occasionally but typically regresses spontaneously after delivery. Most evidence indicates that pregnancy does not incite tooth decay.
Hemorrhoids are fairly common during pregnancy. They are caused in large measure by constipation and the elevated pressure in veins below the level of the enlarged uterus.
Although the liver in some animals increases remarkably in size during pregnancy, there is no evidence for such an increase during human pregnancy (Combes and Adams, 1971) Histological evaluation of liver biopsies, including examination with the electron microscope, have shown no distinct changes in liver morphology in normal pregnant women (Ingerslev and Teilum, 1946)
Some of the laboratory tests used to evaluate hepatic function yield appreciably different results during normal pregnancy. Moreover, some of those changes are similar to those in patients with liver disease. Total alkaline phosphatase activity in serum almost doubles during normal pregnancy, but much of the increase is attributable to heat-stable placental alkaline phosphatase isozymes. Serum aspartate transaninase (AST), alanine transaminase (ALT), gamma glutamyl transferase (GGT), and bilirubin levels are slightly lower during pregnancy
Mendenhall (1970) reconfirmed decreased plasma albumin concentration, showing it to average 3.0 g/dL late in pregnancy compared with 4.3 g/dL in nonpregnant women. Total albumin is increased, however, because of a greater volume of distribution. The reduction in albumin concentrations, combined with a normal slight increase in plasma globulins, results in a decrease in the albumin-to-globulin ratio similar to that seen in certain hepatic diseases.
Plasma cholinesterase activity is reduced during normal pregnancy. The magnitude of the decrease is about the same as the decrease in the concentration of albumin (Kambam and associates, 1988; Pritchard, 1955). Leucine aminopeptidase activity is markedly elevated in serum from pregnant women. The increase results from the appearance of a pregnancy-specific enzyme (or enzymes) with distinct substrate specificities (Song and Kappas, 1968). Pregnancy-induced aminopeptidase has oxytocinase and vasopressinase activity.
There is considerable alteration of gallbladder function during pregnancy Braverman and co-workers (1980), using ultrasonography, found impaired gallbladder contraction and high residual volume It has been suggested that progesterone impairs gallbladder contraction by inhibiting cholecystokinin-mediated smooth muscle stimulation, the primary regulator of gallbladder contraction Impaired gallbladder contraction leads to stasis, and this, associated with the increased cholesterol saturation of pregnancy, at least partially explains the increased prevalence of cholesterol stones in women who have been pregnant many times
The effects of pregnancy on maternal bile acid serum concentrations have been incompletely characterized despite the long-acknowledged propensity for pregnancy to cause intrahepatic cholestasis and pruritus gravidarum from retained bile salts (Chap. 48, p. 1283 and Chap. 54, p. 1431). Cholestasis has been linked to high circulating levels of estrogen, which inhibit intraductal transport of bile acids (Simon and colleagues, 1996). Leslie and associates (2000), however, compared circulating estrogen levels in normal pregnant women with those of women with cholestasis. They found the latter group to have significantly lower plasma estrogen levels as well as impaired fetal production of dehydro-epiandrosterone (DHEA), which is the precursor to placental estrogen production. The significance of this recent finding is unclear.
THE BONY PELVIS
In both women and men the pelvis forms the bony ring through which body weight is transmitted to the lower extremities, but in women it has a special form that adapts it to childbearing. The pelvis is composed of four bones: the sacrum, coccyx, and two innominate bones. Each innominate bone is formed by the fusion of the ilium, ischium, and pubis. The innominate bones are joined to the sacrum at the sacroiliac synchondroses and to one another at the symphysis pubis.
The false pelvis lies above the linea terminalis and the true pelvis below this anatomical boundary (Fig. 3-20). The false pelvis is bounded posteriorly by the lumbar vertebrae and laterally by the iliac fossae, and in front the boundary is formed by the lower portion of the anterior abdominal wall.
The true pelvis is the portion important in childbearing. It is bounded above by the promontory and alae of the sacrum, the linea terminalis, and the upper margins of the pubic bones, and below by the pelvic outlet. The cavity of the true pelvis can be described as an obliquely truncated, bent cylinder with its greatest height posteriorly, because its anterior wall at the symphysis pubis measures about 5 cm and its posterior wall about 10 cm . With the woman upright, the upper portion of the pelvic canal is directed downward and backward, and its lower course curves and becomes directed downward and forward.
The walls of the true pelvis are partly bony and partly ligamentous. The posterior boundary is the anterior surface of the sacrum, and the lateral limits are formed by the inner surface of the ischial bones and the sacrosciatic notches and ligaments. In front the true pelvis is bounded by the pubic bones, the ascending superior rami of the ischial bones, and the obturator foramina.
The sidewalls of the true pelvis of the normal adult woman converge somewhat; therefore, if the planes of the ischial bones were extended downward, they would meet near the knee. Extending from the middle of the posterior margin of each ischium are the ischial spines. The ischial spines are of great obstetrical importance because the distance between them usually represents the shortest diameter of the pelvic cavity. They also serve as valuable landmarks in assessing the level to which the presenting part of the fetus has descended into the true pelvis
The sacrum forms the posterior wall of the pelvic cavity. Its upper anterior margin corresponds to the body of the first sacral vertebra and is designated as the promontory. The promontory may be felt on vaginal examination in small pelves and can provide a landmark for clinical pelvimetry. Normally the sacrum has a marked vertical and a less pronounced horizontal concavity, which in abnormal pelves may undergo important variations. A straight line drawn from the promontory to the tip of the sacrum usually measures 10 cm, whereas the distance along the concavity averages 12 cm.
The descending inferior rami of the pubic bones unite at an angle of 90 to 100 degrees to form a rounded arch under which the fetal head must pass.
SYMPHYSIS PUBIS. Anteriorly, the pelvic bones are joined together by the symphysis pubis. This structure consists of fibrocartilage and the superior and inferior pubic ligaments; the latter is frequently designated the arcuate ligament of the pubis (Fig. 3-23). The symphysis has a certain degree of mobility, which increases during pregnancy. This fact was demonstrated by Budin (1897), who reported that if a finger was inserted into the vagina of a pregnant woman and she then walked, the ends of the pubic bones could be felt moving up and down with each step.
SACROILIAC JOINTS. Posteriorly the pelvic bones are joined by the articulations between the sacrum and the iliac portion of the innominate bones (sacroiliac joints). These joints also have a certain degree of mobility.
RELAXATION OF THE PELVIC JOINTS. During pregnancy, relaxation of these joints likely results from hormonal changes. Abramson and co-workers (1934) observed that relaxation of the symphysis pubis commenced in women in the first half of pregnancy and increased during the last 3 months. These investigators reported that regression of relaxation began immediately after parturition and was completed within 3 to 5 months. The symphysis pubis also increases in width during pregnancy (more in multiparas than in primigravidas), and returns to normal soon after delivery. By careful radiographic studies, Borell and Fernstrom (1957) demonstrated that the rather marked mobility of the pelvis of women at term was caused by an upward gliding movement of the sacroiliac joint. The displacement, which is greatest in the dorsal lithotomy position, may increase the diameter of the outlet by 1.5 to 2.0 cm. This is the main justification for placing a woman in this position for a vaginal delivery. It should be noted, however, that the increase in the diameter of the pelvic outlet occurs only if the sacrum is allowed to rotate posteriorly, that is, only if the sacrum is not forced anteriorly by the weight of the maternal pelvis against the delivery table or bed (Russell, 1969, 1982). This is likely the reason that the McRoberts maneuver often is successful in releasing an obstructed shoulder in a case of shoulder dystocia (Chap. 19, p. 461). Gardosi and co-workers (1989) reported in a randomized controlled trial that a modified squatting position in the second stage of labor resulted in a shorter second stage and fewer perineal lacerations. However, there was an increase in labial lacerations. The authors attributed the "success" of the method to increasing the interspinous diameter and the diameter of the pelvic outlet (Russell, 1969, 1982) as well as to improving the "pushing efforts" of the laboring woman. Although these observations are unconfirmed, such a squatting position is assumed by many primitive women as the usual position for birth (Gardosi and associates, 1989; Russell, 1982).
PLANES AND DIAMETERS OF THE PELVIS
Because of its complex shape, it is difficult to describe the exact location of an object within the pelvis. For convenience, therefore, the pelvis is described as having four imaginary planes:
1. The plane of the pelvic inlet
2. The plane of the midpelvis (least pelvic dimensions).
3. The plane of greatest pelvic dimensions.
4. The plane of the pelvic outlet
Because this last plane has no obstetrical significance, it is not considered further.
PELVIC INLET. The pelvic inlet (superior strait) is bounded posteriorly by the promontory and alae of the sacrum, laterally by the linea terminalis, and anteriorly by the horizontal rami of the pubic bones and symphysis pubis. The configuration of the inlet of the human female pelvis typically is more nearly round than ovoid. Caldwell and co-workers (1934) identified radiographically a nearly round or gynecoid pelvic inlet in approximately 50 percent of the pelves of white women.
Four diameters of the pelvic inlet are usually described: anteroposterior, transverse, and two obliques. The obstetrically important anteroposterior diameter is the shortest distance between the promontory of the sacrum and the symphysis pubis, and is designated the obstetrical conjugate. Normally, the obstetrical conjugate measures 10 cm or more, but it may be considerably shortened in abnormal pelves.
The transverse diameter is constructed at right angles to the obstetrical conjugate and represents the greatest distance between the linea terminalis on either side. It usually intersects the obstetrical conjugate at a point about 4 cm in front of the promontory. The segment of the obstetrical conjugate from the intersection of these two lines to the promontory is designated the posterior sagittal diameter of the inlet.
Each of the oblique diameters extends from one of the sacroiliac synchondroses to the iliopectineal eminence on the opposite side of the pelvis. They average just under 13 cm and are designated right and left, according to whether they originate at the right or left sacroiliac synchondrosis.
The anteroposterior diameter of the pelvic inlet that has been identified as the true conjugate does not represent the shortest distance between the promontory of the sacrum and symphysis pubis. The shortest distance is the obstetrical conjugate, which is the shortest anteroposterior diameter through which the head must pass in descending through the pelvic inlet.
The obstetrical conjugate cannot be measured directly with the examining fingers; therefore, various instruments have been designed in an effort to obtain such a measurement. Unfortunately, none of these instruments has proven to be reliable. For clinical purposes, it is sufficient to estimate the length of the obstetrical conjugate indirectly. This is accomplished by measuring the distance from the lower margin of the symphysis to promontory of the sacrum, that is, the diagonal conjugate), and subtracting 1.5 to 2 cm from the result, according to the height and inclination of the symphysis pubis
MIDPELVIS. The midpelvis at the level of the ischial spines (midplane, or plane of least pelvic dimensions) is of particular importance following engagement of the fetal head in obstructed labor. The interspinous diameter, 10 cm or somewhat more, is usually the smallest diameter of the pelvis. The anteroposterior diameter, through the level of the ischial spines, normally measures at least 11.5 cm. The posterior component (posterior sagittal diameter), between the sacrum and the line created by the interspinous diameter, is usually at least 4.5 cm.
PELVIC OUTLET. The outlet of the pelvis consists of two approximately triangular areas not in the same plane but having a common base, which is a line drawn between the two ischial tuberosities (Fig. 3-26). The apex of the posterior triangle is at the tip of the sacrum, and the lateral boundaries are the sacrosciatic ligaments and the ischial tuberosities. The anterior triangle is formed by the area under the pubic arch. Three diameters of the pelvic outlet usually are described: the anteroposterior, transverse, and posterior sagittal. The anteroposterior diameter (9.5 to 11.5 cm) extends from the lower margin of the symphysis pubis to the tip of the sacrum. The transverse diameter (11 cm) is the distance between the inner edges of the ischial tuberosities. The posterior sagittal diameter extends from the tip of the sacrum to a right-angle intersection with a line between the ischial tuberosities. The normal posterior sagittal diameter of the outlet usually exceeds 7.5 cm
In obstructed labors caused by a narrowing of the midpelvis or pelvic outlet, the prognosis for vaginal delivery often depends on the length of the posterior sagittal diameter of the pelvic outlet.
In the past, x-ray pelvimetry was used frequently in women with suspected cephalopelvic disproportion or fetal malpresentation. Pelvic radiography also was used as an aid in understanding the general architecture and configuration of the pelvis, as well as its size. Caldwell and Moloy (1933, 1934) developed a classification of the pelvis that is still used. The classification is based upon the shape of the pelvis, and familiarity with the classification helps the physician to understand the mechanisms of labor in normally and abnormally shaped pelves.
PELVIC SIZE AND ITS CLINICAL ESTIMATION
PELVIC INLET MEASUREMENTS DIAGONAL CONJUGATE. In many abnormal pelves, the anteroposterior diameter of the pelvic inlet (the obstetrical conjugate) is considerably shortened. It is important therefore to determine its length, but this measurement can be obtained only by radiographic techniques. The distance from the sacral promontory to the lower margin of the symphysis pubis (the diagonal conjugate), however, can be measured clinically. The examiner introduces two fingers into the vagina; before measuring the diagonal conjugate, the mobility of the coccyx is evaluated and the anterior surface of the sacrum is palpated. The mobility of the coccyx is tested by palpating it with the fingers in the vagina and attempting to move it to and fro. The anterior surface of the sacrum is then palpated from below upward, and its vertical and lateral curvatures are noted. In normal pelves only the last three sacral vertebrae can be felt without indenting the perineum, whereas in markedly contracted pelves the entire anterior surface of the sacrum usually is readily accessible. Occasionally, the mobility of the coccyx and the anatomical features of the lower sacrum may be defined more easily by rectal examination.
Except in extreme degrees of pelvic contraction, in order to reach the promontory of the sacrum, the examiner's elbow must be depressed and, unless the examiner's fingers are unusually long, the perineum forcibly indented by the knuckles of the examiner's third and fourth fingers. The index and the second fingers, held firmly together, are carried up and over the anterior surface of the sacrum. By sharply depressing the wrist, the promontory may be felt by the tip of the second finger as a projecting bony margin. With the finger closely applied to the most prominent portion of the upper sacrum, the vaginal hand is elevated until it contacts the pubic arch; and the immediately adjacent point on the index finger is marked. The hand is withdrawn, and the distance between the mark and the tip of the second finger is measured. The diagonal conjugate is determined, and the obstetrical conjugate is computed by subtracting 1.5 to 2.0 cm, depending upon the height and inclination of the symphysis pubis. If the diagonal conjugate is greater than 11.5 cm, it is justifiable to assume that the pelvic inlet is of adequate size for vaginal delivery of a normal-sized fetus.
Transverse contraction of the inlet can be measured only by imaging pelvimetry. Such a contraction is possible even in the presence of an adequate anteroposterior diameter.
ENGAGEMENT. This refers to the descent of the biparietal plane of the fetal head to a level below that of the pelvic inlet (Figs. 3-30 and 3-31). When the biparietal, or largest, diameter of the normally flexed fetal head has passed through the inlet, the head is engaged. Although engagement of the fetal head usually is regarded as a phenomenon of labor, in nulliparas it commonly occurs during the last few weeks of pregnancy. When it does so, it is confirmatory evidence that the pelvic inlet is adequate for that fetal head. With engagement, the fetal head serves as an internal pelvimeter to demonstrate that the pelvic inlet is ample for that fetus.
Whether the head is engaged may be ascertained by rectal or vaginal examination or by abdominal palpation. After gaining experience with vaginal examination, it becomes relatively easy to locate the station of the lowermost part of the fetal head in relation to the level of the ischial spines. If the lowest part of the occiput is at or below the level of the spines, the head usually, but not always, is engaged, because the distance from the plane of the pelvic inlet to the level of the ischial spines is approximately 5 cm in most pelves, and the distance from the biparietal plane of the unmolded fetal head to the vertex is about 3 to 4 cm. Under these circumstances, the vertex cannot possibly reach the level of the spines unless the biparietal diameter has passed the inlet, or unless there has been considerable elongation of the fetal head because of molding and formation of a caput succedaneum.
Engagement may be ascertained less satisfactorily by abdominal examination. If the biparietal plane of a term-sized infant has descended through the inlet, the examining fingers cannot reach the lowermost part of the head. Thus, when pushed downward over the lower abdomen, the examining fingers will slide over that portion of the head proximal to the biparietal plane (nape of the neck) and diverge. Conversely, if the head is not engaged, the examining fingers can easily palpate the lower part of the head and will converge
Fixation of the fetal head is its descent through the pelvic inlet to a depth that prevents its free movement in any direction when pushed by both hands placed over the lower abdomen. Fixation is not necessarily synonymous with engagement. Although a head that is freely movable on abdominal examination cannot be engaged, fixation of the head is sometimes seen when the biparietal plane is still 1 cm or more above the pelvic inlet, especially if the head is molded appreciably.
Although engagement is conclusive evidence of an adequate pelvic inlet for that fetal head, its absence is by no means always indicative of pelvic contraction.
PELVIC OUTLET MEASUREMENTS. An important dimension of the pelvic outlet that is accessible for clinical measurement is the diameter between the ischial tuberosities, variously called the biischial diameter, intertuberous diameter, and transverse diameter of the outlet. A measurement of over 8 cm is considered normal. The measurement of the transverse diameter of the outlet can be estimated by placing a closed fist against the perineum between the ischial tuberosities, after first measuring the width of the closed fist. Usually the closed fist is wider than 8 cm. The shape of the subpubic arch also can be evaluated at the same time by palpating the pubic rami from the subpubic region toward the ischial tuberosities.
MIDPELVIS ESTIMATION. Clinical estimation of midpelvis capacity by any direct form of measurement is not possible. If the ischial spines are quite prominent, the sidewalls are felt to converge, and the concavity of the sacrum is very shallow; if the biischial diameter of the outlet is less than 8 cm, then suspicion is aroused about a contraction in this region.
From an obstetrical viewpoint, the size of the fetal head is important because an essential feature of labor is the adaptation between the fetal head and the maternal bony pelvis. Only a comparatively small part of the head at term is represented by the face; the rest is composed of the firm skull, which is made up of two frontal, two parietal, and two temporal bones, along with the upper portion of the occipital bone and the wings of the sphenoid.
These bones are not united rigidly, but rather are separated by membranous spaces, called sutures The most important sutures are the frontal, between the two frontal bones; the sagittal, between the two parietal bones; the two coronal, between the frontal and parietal bones; and the two lambdoid, between the posterior margins of the parietal bones and upper margin of the occipital bone. With a vertex presentation, all of the sutures are palpable during labor except the temporal sutures, which are situated on either side between the inferior margin of the parietal and upper margin of the temporal bones, covered by soft parts, and cannot be felt in the living fetus.
Where several sutures meet, an irregular space forms, which is enclosed by a membrane and designated as a fontanel (Fig. 7-6). The three most clinically important fontanels are the greater, the lesser, and the temporal fontanels. The greater or anterior fontanel is a lozenge-shaped space that is situated at the junction of the sagittal and the coronal sutures. The lesser or posterior fontanel is represented by a small triangular area at the intersection of the sagittal and lambdoid sutures. Both can be palpated readily during labor. The localization of these fontanels gives important information concerning the presentation and position of the fetus. The temporal, or casserian fontanels, situated at the junction of the lambdoid and temporal sutures, have no diagnostic significance.
It is customary to measure certain critical diameters and circumferences of newborn head (Fig. 7-7).
The diameters most frequently used, and the average lengths thereof, are as follows:
1. The occipitofrontal (11.5 cm), which follows a line extending from a point just above the root of the nose to the most prominent portion of the occipital bone.
2. The biparietal (9.5 cm), the greatest transverse diameter of the head, which extends from one parietal boss to the other.
3. The bitemporal (8.0 cm), the greatest distance between the two temporal sutures.
4. The occipitomental (12.5 cm), from the chin to the most prominent portion of the occiput.
5. The suboccipitobregmatic (9.5 cm), which follows a line drawn from the middle of the large fontanel to the undersurface of the occipital bone just where it joins the neck ).
The greatest circumference of the head, which corresponds to the plane of the occipitofrontal diameter, averages 34.5 cm, a size too large to fit through the pelvis without flexion. The smallest circumference, corresponding to the plane of the suboccipitobregmatic diameter, is 32 cm. As a rule, white infants have larger heads than nonwhite infants; boys, somewhat larger than girls; and the infants of multiparas, larger heads than those of nulliparas. The bones of the cranium are normally connected only by a thin layer of fibrous tissue which allows considerable shifting or sliding of each bone to accommodate the size and shape of the maternal pelvis. This intrapartum process is termed molding. Because of the varying mobility of the bones of the skull and varying presentations of the fetal head relative to the pelvis, a variety of newborn head shapes are possible. The head position and degree of skull ossification result in a spectrum of cranial plasticity from minimal to great and, in some cases, undoubtedly contributes to fetopelvic disproportion, a leading indication for cesarean delivery (Chap. 18, p. 440).
FETAL BRAIN. There is a steady gestational age-related change in the appearance and function of the fetal brain (Fig. 7-8). It is therefore possible to identify fetal age rather precisely from its external appearance (Dolman, 1977). Myelination of the ventral roots of the cerebrospinal nerves and brainstem begins at approximately 6 months, but the major portion of myelination occurs after birth. The lack of myelin and the incomplete ossification of the fetal skull permit the structure of the brain to be seen with ultrasound throughout gestation.
LIE, PRESENTATION, ATTITUDE, AND POSITION
By convention, fetal orientation is described with respect to fetal lie, presentation, attitude, and position. These can be established clinically by abdominal palpation, vaginal examination, and auscultation, or by technical means using sonography or x-ray. Clinical assessment is less accurate, or even sometimes impossible to perform and interpret in obese women.
The lie is the relation of the long axis of the fetus to that of the mother, and is either longitudinal or transverse.
Occasionally, the fetal and the maternal axes may cross at a 45-degree angle, forming an oblique lie, which is unstable and always becomes longitudinal or transverse during the course of labor. Longitudinal lies are present in over 99 percent of labors at term. Predisposing factors for transverse lies include multiparity, placenta previa, hydramnios, and uterine anomalies (Gemer and Segal, 1994).
The presenting part is that portion of the body of the fetus that is either foremost within the birth canal or in closest proximity to it.
The presenting part can be felt through the cervix on vaginal examination. The presenting part determines the presentation. Accordingly, in longitudinal lies, the pre-senting part is either the fetal head or breech, creating cephalic and breech presentations, respectively. When the fetus lies with the long axis transversely, the shoulder is the presenting part. Thus, a shoulder presentation is felt through the cervix on vaginal examination.
CEPHALIC PRESENTATION. These are classified according to the relation of the head to the body of the fetus (Fig. 12-1). Ordinarily the head is flexed sharply so that the chin is in contact with the thorax. In this circumstance, the occipital fontanel is the presenting part, and such a presentation is usually referred to as a vertex or occiput presentation. Actually, the vertex lies just in front of the occipital fontanel, and the occiput just behind the fontanel, as illustrated in Figure 7-7 (p. 136). Much less commonly, the fetal neck may be sharply extended so that the occiput and back come in contact and the face is foremost in the birth canal—face presentation. The fetal head may assume a position between these extremes, partially flexed in some cases, with the anterior (large) fontanel, or bregma, presenting (sinciput presentation), or partially extended in other cases, with the brow presenting (brow presentation). These latter two presentations are usually transient. As labor progresses, sinciput and brow presentations are almost always converted into vertex or face presentations by flexion or extension, respectively.
BREECH PRESENTATION. When the fetus presents as a breech, there are three general configurations. When the thighs are flexed and the legs extended over the anterior surfaces of the body, this is termed a frank breech presentation (Fig. 12-2). If the thighs are flexed on the abdomen and the legs upon the thighs, this is a complete breech presentation (Fig. 12-3). If one or both feet, or one or both knees, are lowermost, then there is an incomplete, or footling, breech presentation (Fig. 12-4).
FETAL ATTITUDE OR POSTURE
In the later months of pregnancy the fetus assumes a characteristic posture described as attitude or habitus As a rule, the fetus forms an ovoid mass that corresponds roughly to the shape of the uterine cavity. The fetus becomes folded or bent upon itself in such a manner that the back becomes markedly convex; the head is sharply flexed so that the chin is almost in contact with the chest; the thighs are flexed over the abdomen; the legs are bent at the knees; and the arches of the feet rest upon the anterior surfaces of the legs. In all cephalic presentations, the arms are usually crossed over the thorax or become parallel to the sides, and the umbilical cord lies in the space between them and the lower extremities. This characteristic posture results from the mode of growth of the fetus and its accommodation to the uterine cavity.
Abnormal exceptions to this attitude occur as the fetal head becomes progressively more extended from the vertex to the face presentation. This results in a progressive change in fetal attitude from a convex (flexed) to a concave (extended) contour of the vertebral column.
Position refers to the relation of an arbitrarily chosen portion of the fetal presenting part to the right or left side of the maternal birth canal. Accordingly, with each presentation there may be two positions, right or left. The fetal occiput, chin (mentum), and sacrum are the determining points in vertex, face, and breech presentations, respectively
VARIETIES OF PRESENTATION AND POSITION
For still more accurate orientation, the relation of a given portion of the presenting part to the anterior, transverse, or posterior portion of the maternal pelvis is considered. Because there are two positions, it follows that there must be three varieties for each position (either right or left), and six varieties for each presentation (three right and three left). Because the presenting part may be in either the left or right position, there are left and right occipital, left and right mental, and left and right sacral presentations, abbreviated as LO and RO, LM and RM, and LS and RS, respectively. Because the presenting part in each of the two positions may be directed anteriorly (A), transversely (T), or posteriorly (P), there are six varieties of each of these three presentations. Thus, in an occiput presentation, the presentation, position, and variety may be abbreviated in clockwise fashion as:
In shoulder presentations, the acromion (scapula) is the portion of the fetus arbitrarily chosen for orientation with the maternal pelvis. One example of the terminology sometimes employed for the purpose is illustrated in Figure 12-9. The acromion or back of the fetus may be directed either posteriorly or anteriorly and superiorly or inferiorly (Chap. 19, p. 455). Because it is impossible to differentiate exactly the several varieties of shoulder presentation by clinical examination, and because such differentiation serves no practical purpose, it is customary to refer to all transverse lies simply as shoulder presentations. Another term used is transverse lie, with back up or back down.
FREQUENCY OF THE VARIOUS PRESENTATIONS AND POSITIONS
At or near term the incidence of the various presentations is approximately as follows: vertex, 96 percent; breech, 3.5 percent; face, 0.3 percent; and shoulder, 0.4 percent. About two thirds of all vertex presentations are in the left occiput position, and a third in the right.
Although the incidence of breech presentation is only a little over 3 percent at term (see Table 19-1), it is much greater earlier in pregnancy. Scheer and Nubar (1976), using ultrasonography, found the incidence of breech presentation to be 14 percent between 29 and 32 weeks' gestation. Subsequently, the breech converted spontaneously to vertex in increasingly higher percentages as term approached.
There are several explanations of why the term fetus usually presents by the vertex. The most logical is because the uterus is piriform shaped. Although the fetal head at term is slightly larger than the breech, the entire podalic pole of the fetus—that is, the breech and its flexed extremities—is bulkier and more movable than the cephalic pole. The cephalic pole is comprised of the fetal head only.
Until about 32 weeks, the amnionic cavity is large compared with the fetal mass, and there is no crowding of the fetus by the uterine walls. At approximately this time, however, the ratio of amnionic fluid volume to fetal mass becomes altered by relative diminution of amnionic fluid and by increasing fetal size. As a result, the uterine walls are apposed more closely to the fetal parts. The fetal lie then is more nearly dependent upon the piriform shape of the uterus. The fetus, if presenting by the breech, often changes polarity in order to make use of the roomier fundus for its bulkier and more movable podalic pole. The high incidence of breech presentation in hydrocephalic fetuses is in accord with this theory, because in this circumstance the cephalic pole of the fetus is larger than the podalic pole.
The cause of breech presentation may be some circumstance that prevents the normal version from taking place, for example, a septum that protrudes into the uterine cavity. A peculiarity of fetal attitude, particularly extension of the vertebral column as seen in frank breeches, may also prevent the fetus from turning. If the placenta implants in the lower uterine segment, normal intrauterine anatomy is distorted. Also, any condition contributing to an abnormality of fetal muscle tone or movement may predispose to persistent breech presentations.
DIAGNOSIS OF FETAL PRESENTATION AND POSITION
Several methods can be used to diagnose fetal presentation and position. These include abdominal palpation, vaginal examination, combined examination, auscultation, and in certain doubtful cases, imaging studies such as ultrasonography, computerized tomographic scans (CT), or magnetic resonance imaging (MRI) studies.
ABDOMINAL PALPATION—LEOPOLD MANEUVERS
Abdominal examination should be conducted systematically employing the four maneuvers described by Leopold and Sporlin in 1894. The mother should be supine and comfortably positioned with her abdomen bared. During the first three maneuvers, the examiner stands at the side of the bed that is most convenient and faces the patient; the examiner reverses this position and faces her feet for the last maneuver. These maneuvers may be difficult if not impossible to perform and interpret if the patient is obese or if the placenta is anteriorly implanted.
FIRST MANEUVER. After outlining the contour of the uterus and ascertaining how nearly the fundus approaches the xiphoid cartilage, the examiner gently palpates the fundus with the tips of the fingers of both hands in order to define which fetal pole is present in the fundus. The breech gives the sensation of a large, nodular body, whereas the head feels hard and round and is more freely movable and ballottable.
SECOND MANEUVER. After determination of the pole that lies in the fundus, the palms are placed on either side of the abdomen, and gentle but deep pressure is exerted. On one side, a hard, resistant structure is felt, the back; and on the other, numerous small, irregular and mobile parts are felt, the fetal extremities. In women with a thin abdominal wall, the fetal extremities can often be differentiated, but in heavier women, only these irregular nodulations may be felt. In the presence of obesity or considerable amnionic fluid, the back is felt more easily by exerting deep pressure with one hand while counter-palpating with the other. By next noting whether the back is directed anteriorly, transversely, or posteriorly, a more accurate picture of the orientation of the fetus is obtained.
THIRD MANEUVER. Using the thumb and fingers of one hand, the lower portion of the maternal abdomen is grasped just above the symphysis pubis. If the presenting part is not engaged, a movable body will be felt, usually the head. The differentiation between head and breech is made as in the first maneuver. If the presenting part is not engaged, all that remains to be defined is the attitude of the head. If by careful palpation it can be shown that the cephalic prominence is on the same side as the small parts, the head must be flexed, and therefore the vertex is the presenting part. When the cephalic prominence of the fetus is on the same side as the back, the head must be extended. If the presenting part is deeply engaged, however, the findings from this maneuver are simply indicative that the lower fetal pole is fixed in the pelvis; the details are then defined by the last (fourth) maneuver.
FOURTH MANEUVER. The examiner faces the mother's feet and, with the tips of the first three fingers of each hand, exerts deep pressure in the direction of the axis of the pelvic inlet. If the head presents, one hand is arrested sooner than the other by a rounded body, the cephalic prominence, while the other hand descends more deeply into the pelvis. In vertex presentations, the prominence is on the same side as the small parts; and in face presentations, on the same side as the back. The ease with which the prominence is felt is indicative of the extent to which descent has occurred. In many instances, when the head has descended into the pelvis, the anterior shoulder may be differentiated readily by the third maneuver. In breech presentations, the information obtained from this maneuver is less precise.
Abdominal palpation can be performed throughout the latter months of pregnancy and during and between the contractions of labor. The findings provide information about the presentation and position of the fetus and the extent to which the presenting part has descended into the pelvis. For example, so long as the cephalic prominence is readily palpable, the vertex has not descended to the level of the ischial spines. The degree of cephalopelvic disproportion, moreover, can be gauged by evaluating the extent to which the anterior portion of the fetal head overrides the symphysis pubis. With experience, it is possible to estimate the size of the fetus, and even to map out the presentation of the second fetus in a twin gestation. According to Lydon-Rochelle and colleagues (1993), experienced clinicians accurately identify fetal malpresentation using Leopold maneuvers with a high sensitivity (88 percent), specificity (94 percent), positive predictive value (74 percent), and negative predictive value (97 percent).
Before labor, the diagnosis of fetal presentation and position by vaginal examination is often inconclusive, because the presenting part must be palpated through a closed cervix and lower uterine segment. With the onset of labor and after cervical dilatation, important information may be obtained. In vertex presentations, the position and variety are recognized by differentiation of the various sutures and fontanels. Face presentations are identified by the differentiation of the portions of the face. Breech presentations are identified by palpation of the sacrum and maternal ischial tuberosities.
In attempting to determine presentation and position by vaginal examination, it is advisable to pursue a definite routine, comprised of four maneuvers:
1. After the woman is prepared appropriately, two fingers of either gloved hand are introduced into the vagina and carried up to the presenting part. The differentiation of vertex, face, and breech is then accomplished readily.
2. If the vertex is presenting, the fingers are introduced into the posterior aspect of the vagina. The fingers are then swept forward over the fetal head toward the maternal symphysis . During this movement, the fingers necessarily cross the fetal sagittal suture. When it is felt, its course is outlined, with small and large fontanels at the opposite ends.
3. The positions of the two fontanels then are ascertained. The fingers are passed to the most anterior extension of the sagittal suture, and the fontanel encountered there is examined carefully and identified; then by a circular motion, the fingers are passed around the side of the head until the other fontanel is felt and differentiated .
4. The station, or extent to which the presenting part has descended into the pelvis, can also be established at this time .
Using these maneuvers, the various sutures and fontanels are located readily, and the possibility of error is lessened considerably. In face and breech presentations, errors are minimized, because the various parts are distinguished more readily.
While auscultation alone does not provide reliable information concerning fetal presentation and position, auscultatory findings sometimes reinforce results obtained by palpation. Ordinarily, fetal heart sounds are transmitted through the convex portion of the fetus that lies in intimate contact with the uterine wall. Therefore, fetal heart sounds are heard best through the fetal back in vertex and breech presentations, and through the fetal thorax in face presentations. The region of the abdomen in which fetal heart sounds are heard most clearly varies according to the presentation and the extent to which the presenting part has descended. In cephalic presentations, fetal heart sounds are best heard midway between the maternal umbilicus and the anterior superior spine of her ilium. In breech presentations, fetal heart tones are usually heard at or slightly above the umbilicus. In occipitoanterior positions, heart sounds usually are heard best a short distance from the midline; in the transverse varieties, they are heard more laterally; and in the posterior varieties, they are best heard well back in the flank.
Improvements in ultrasonic techniques have provided another diagnostic aid of particular value in doubtful cases. In obese women or in women whose abdominal walls are rigid, a sonographic examination may provide information to solve many diagnostic problems and lead to early recognition of a breech or shoulder presentation that might otherwise have escaped detection until late in labor. Employing ultrasonography, the fetal head and body can be located without the potential hazards of radiation. In some clinical situations, the information obtained radiographically far exceeds the minimal risk from a single x-ray exposure
PHYSIOLOGY OF DELIVERY. ANESTHESIA IN LABOR. PHYSIOLOGY OF POST-NATAL PERIOD. PHYSIOLOGY OF INFANTS’ PERIOD
CLINICAL COURSE OF LABOR
The last few hours of human pregnancy are characterized by uterine contractions that effect dilatation of the cervix and force the fetus through the birth canal. Much energy is expended during this time; hence the use of the term labor to describe this process. The myometrial contractions of labor are painful, which is why the term labor pains is used to describe this process.
Before these forceful, painful contractions begin, however, the uterus must be prepared for labor. During the first 36 to 38 weeks of gestation, the myometrium is unresponsive; after this prolonged period of quiescence, a transitional phase is required during which myometrial unresponsiveness is suspended and the cervix is softened and effaced. Indeed, there are multiple functional states of the uterus that must be implemented during pregnancy and the puerperium; these are described later and categorized as the uterine phases of parturition.
Myometrial contractions that do not cause cervical dilatation may be observed at any time during pregnancy. These contractions are characterized by unpredictability in occurrence, lack of intensity, and brevity of duration. Any discomfort that they produce is usually confined to the lower abdomen and groin. Near the end of pregnancy, as the uterus undergoes preparation for labor, contractions of this type are more common, especially in multiparas, and sometimes are referred to as false labor. In some women, however, the forceful uterine contractions that effect cervical dilatation, fetal descent, and delivery of the conceptus begin suddenly, seemingly without warning.
ANATOMICAL AND PHYSIOLOGICAL CONSIDERATIONS. There are unique characteristics of myometrial muscle (and other smooth muscles) compared with skeletal muscle. Huszar and Walsh (1989) point out that these differences create a peculiar advantage for the myometrium in the efficiency of uterine contractions and the delivery of the fetus. First, the degree of shortening of smooth muscle cells with contractions may be one order of magnitude greater than that attained in striated muscle cells. Second, forces can be exerted in smooth muscle cells in any direction, whereas the contraction force generated by skeletal muscle is always aligned with the axis of the muscle fibers. Third, smooth muscle is not organized in the same manner as skeletal muscle; in myometrium, the thick and thin filaments are found in long, random bundles throughout the cells. This arrangement facilitates greater shortening and force-generating capacity of smooth muscle. Fourth, there is the advantage that multidirectional force generation in myometrial smooth muscle permits versatility in expulsive force directionality that can be brought to bear irrespective of the lie or presentation of the fetus.
BIOCHEMISTRY OF SMOOTH MUSCLE CONTRACTIONS. The interaction of myosin and actin is essential to muscle contraction. Myosin (Mr about 500,000) is comprised of multiple light and heavy chains and is laid down in thick myofilaments. The interaction of myosin and actin, which causes activation of ATPase, ATP hydrolysis, and force generation, is effected by enzymatic phosphorylation of the 20-kd light chain of myosin (Stull and colleagues, 1988, 1998). This phosphorylation reaction is catalyzed by the enzyme myosin light chain kinase, which is activated by Ca2+ (Fig. 11-1).
Ca2+ binds to calmodulin, a calcium-binding regulatory protein, which in turn binds to and activates myosin light chain kinase. In this manner, agents that act on myometrial smooth muscle cells to cause an increase in the intracellular cytosolic concentration of calcium ([Ca2+]i) promote contraction. Conditions that cause a decrease in [Ca2+]i favor relaxation. Ordinarily, agents that cause an increase in the intracellular concentration of cyclic adenosine monophosphate (cAMP) or cyclic guanosine monophosphate (cGMP) promote uterine relaxation. It is believed that cAMP and cGMP act to cause a decrease in [Ca2+]i, although the exact mechanism(s) is not defined. The biochemistry and physiology of smooth muscle contractility have been reviewed by Barany and Barany (1990) and by Sanborn and colleagues (1994).
THE THREE STAGES OF LABOR. Active labor is divided into three separate stages.
The first stage of labor begins when uterine contractions of sufficient frequency, intensity, and duration are attained to bring about effacement and progressive dilatation of the cervix. The first stage of labor ends when the cervix is fully dilated, that is, when the cervix is sufficiently dilated (about 10 cm) to allow passage of the fetal head. The first stage of labor, therefore, is the stage of cervical effacement and dilatation.
The second stage of labor begins when dilatation of the cervix is complete, and ends with delivery of the fetus. The second stage of labor is the stage of expulsion of the fetus.
The third stage of labor begins immediately after delivery of the fetus, and ends with the delivery of the placenta and fetal membranes. The third stage of labor is the stage of separation and expulsion of the placenta.
CLINICAL ONSET OF LABOR. A rather dependable sign of the impending onset of active labor (provided rectal or vaginal examinations have not been performed in the preceding 48 hours) is the discharge of a small amount of blood-tinged mucus from the vagina. This represents the extrusion of the plug of mucus that had filled the cervical canal during pregnancy, and is referred to as "show" or "bloody show." This is a late sign, because commonly labor is already in progress or likely will ensue during the next several hours to few days. Normally, only a few drops of blood escape with the mucus plug; more substantial bleeding is suggestive of an abnormal cause.
UTERINE CONTRACTIONS CHARACTERISTIC OF LABOR. Unique among physiological muscular contractions, those of uterine smooth muscle of labor are painful. The cause of the pain is not known definitely, but several possibilities have been suggested:
1. Hypoxia of the contracted myometrium (as in angina pectoris).
2. Compression of nerve ganglia in the cervix and lower uterus by the interlocking muscle bundles.
3. Stretching of the cervix during dilatation.
4. Stretching of the peritoneum overlying the fundus.
Compression of nerve ganglia in the cervix and lower uterine segment by the contracting myometrium is an especially attractive hypothesis. Paracervical infiltration with a local anesthetic usually produces appreciable relief of pain during subsequent uterine contractions
Uterine contractions are involuntary and, for the most part, independent of extrauterine control. Neural blockage from epidural analgesia does not diminish the frequency and intensity of uterine contractions. Moreover, the myometrial contractions in paraplegic women are normal, though painless, as in women after bilateral lumbar sympathectomy.
Mechanical stretching of the cervix enhances uterine activity in several species, including humans. This phenomenon has been referred to as the Ferguson reflex (1941). The exact mechanism by which mechanical dilatation of the cervix causes increased myometrial contractility is not clear. Release of oxytocin was suggested as the cause, but this is not proven. Manipulation of the cervix and "stripping" the fetal membranes is associated with an increase in the levels of prostaglandin F2a metabolite (PGFM) in blood .
The interval between contractions diminishes gradually from about 10 minutes at the onset of the first stage of labor to as little as 1 minute or less in the second stage. Periods of relaxation between contractions, however, are essential to the welfare of the fetus. Unremitting contraction of the uterus compromises uteroplacental blood flow, and ultimately, fetal-placental flow, sufficiently to cause fetal hypoxemia. In the active phase of labor, the duration of each contraction ranges from 30 to 90 seconds, averaging about 1 minute. There is appreciable variability in the intensity of uterine contractions during apparently normal labor, as emphasized by Schulman and Romney (1970). They recorded the amnionic fluid pressures generated by uterine contractions during spontaneous labor; the average was about 40 mm Hg, but varied from 20 to 60 mm Hg (Chap. 14, p. 355).
DIFFERENTIATION OF UTERINE ACTIVITY. During active labor, the uterus is transformed into two distinct parts. The actively contracting upper segment becomes thicker as labor advances. The lower portion, comprising the lower segment of the uterus and the cervix, is relatively passive compared with the upper segment, and it develops into a much more thinly walled passage for the fetus. The lower uterine segment is analogous to a greatly expanded and thinned-out isthmus of the uterus of nonpregnant women, the formation of which is not solely a phenomenon of labor. The lower segment develops gradually as pregnancy progresses and then thins remarkably during labor (Figs. 11-2 and 11-3). By abdominal palpation, even before rupture of the membranes, the two segments can be differentiated during a contraction. The upper uterine segment is quite firm or hard, whereas the consistency of the lower uterine segment is much less firm. The former is the actively contracting part of the uterus; the latter is the distended, normally much more passive, portion.
If the entire wall of uterine musculature, including the lower uterine segment and cervix, were to contract simultaneously and with equal intensity, the net expulsive force would be decreased markedly. Herein lies the importance of the division of the uterus into an actively contracting upper segment and a more passive lower segment that differ not only anatomically but also physiologically. The upper segment contracts, retracts, and expels the fetus; in response to the force of the contractions of the upper segment, the softened lower uterine segment and cervix dilate and thereby form a greatly expanded, thinned-out muscular and fibromuscular tube through which the fetus can be extruded.
The myometrium of the upper uterine segment does not relax to its original length after contractions; rather, it becomes relatively fixed at a shorter length. The tension, however, remains the same as before the contraction. The upper portion of the uterus, or active segment, contracts down on its diminishing contents, but myometrial tension remains constant. The net effect is to take up slack, maintaining the advantage gained in the expulsion of the fetus, and keeping the uterine musculature in firm contact with the intrauterine contents. As the consequence of retraction, each successive contraction commences where its predecessor left off, so that the upper part of the uterine cavity becomes slightly smaller with each successive contraction. Because of the successive shortening of the muscular fibers with contractions, the upper active uterine segment becomes progressively thickened throughout the first and second stages of labor and tremendously thickened immediately after delivery of the fetus (Fig. 11-2). The phenomenon of retraction of the upper uterine segment is contingent upon a decrease in the volume of its contents. For the contents to be diminished, particularly early in labor when the entire uterus is virtually a closed sac with only a minute opening at the cervical os, the musculature of the lower segment must stretch. This permits increasingly more of the intrauterine contents to occupy the lower segment, and the upper segment retracts only to the extent that the lower segment distends and the cervix dilates.
The relaxation of the lower uterine segment is not a complete relaxation, but rather the opposite of retraction. The fibers of the lower segment become stretched with each contraction of the upper segment, after which these are not returned to the previous length but rather remain relatively fixed at the longer length; the tension, however, remains essentially the same as before. The musculature still manifests tone, still resists stretch, and still contracts somewhat on stimulation. The successive lengthening of the muscular fibers in the lower uterine segment, as labor progresses, is accompanied by thinning, normally to only a few millimeters in the thinnest part. As a result of the thinning of the lower uterine segment and the concomitant thickening of the upper, the boundary between the two is marked by a ridge on the inner uterine surface, the physiological retraction ring. When the thinning of the lower uterine segment is extreme, as in obstructed labor, the ring is very prominent, forming a pathological retraction ring. This is an abnormal condition also known as Bandl ring, which is illustrated in Figure 11-2 and discussed further in Chapter 18 (p. 443). The existence of a gradient of diminishing physiological activity from fundus to cervix was established from measurements of differences in behavior of the upper and lower parts of the uterus during normal labor.
CHANGE IN UTERINE SHAPE. Each contraction produces an elongation of the uterine ovoid with a concomitant decrease in horizontal diameter. By virtue of this change in shape, there are important effects on the process of labor. First, the decrease in horizontal diameter produces a straightening of the fetal vertebral column, pressing its upper pole firmly against the fundus of the uterus, whereas the lower pole is thrust farther downward and into the pelvis. The lengthening of the fetal ovoid thus produced has been estimated as between 5 and 10 cm. The pressure exerted in this fashion is known as the fetal axis pressure. Second, with lengthening of the uterus, the longitudinal fibers are drawn taut and because the lower segment and cervix are the only parts of the uterus that are flexible, these are pulled upward over the lower pole of the fetus. This effect on the musculature of the lower segment and on the cervix is an important factor in cervical dilatation.
ANCILLARY FORCES IN LABOR. After the cervix is dilated fully, the most important force in the expulsion of the fetus is that produced by increased maternal intra-abdominal pressure. This is created by contraction of the abdominal muscles simultaneously with forced respiratory efforts with the glottis closed. This is referred to as "pushing." The nature of the force produced is similar to that involved in defecation, but the intensity usually is much greater. The importance of intra-abdominal pressure in fetal expulsion is most clearly attested to by the labors of women who are paraplegic. Such women suffer no pain, although the uterus may contract vigorously. Cervical dilatation, in large measure the result of uterine contractions acting on a softened cervix, proceeds normally, but expulsion of the infant is accomplished more readily when the woman is instructed to bear down and can do so during a uterine contraction.
Although increased intra-abdominal pressure is required for the spontaneous completion of labor, it is futile until the cervix is fully dilated. Specifically, it is a necessary auxiliary to uterine contractions in the second stage of labor, but pushing accomplishes little in the first stage, except to produce fatigue. Intra-abdominal pressure also may be important in the third stage of labor, especially if the parturient is unattended. After the placenta has separated, its spontaneous expulsion is aided by the mother increasing intra-abdominal pressure.
Before the onset of labor, during the phase of uterine awakening and preparedness, the cervix is softened, which facilitates dilatation of the cervix once forceful myometrial contractions of labor begin.
CHANGES INDUCED IN THE CERVIX WITH LABOR. The effective force of the first stage of labor is the uterine contraction, which in turn exerts hydrostatic pressure through the fetal membranes against the cervix and lower uterine segment. In the absence of intact membranes, the fetal presenting part is forced directly against the cervix and lower uterine segment. As the result of the action of these forces, two fundamental changes—effacement and dilatation—take place in the already softened cervix. For the head of the average fetus at term to pass through the cervix, the cervical canal must dilate to a diameter of about 10 cm; at this time, the cervix is said to be completely (or fully) dilated. There may be no fetal descent during cervical effacement, but most commonly the presenting fetal part descends somewhat as the cervix dilates. During the second stage of labor, descent of the fetal presenting part typically occurs rather slowly but steadily in nulliparas. In multi-paras, however, particularly those of high parity, descent may be very rapid.
The "obliteration" or "taking up" of the cervix is the shortening of the cervical canal from a length of about 2 cm to a mere circular orifice with almost paper-thin edges. This process is referred to as effacement and takes place from above downward. The muscular fibers at about the level of the internal cervical os are pulled upward, or "taken up," into the lower uterine segment, as the condition of the external os remains temporarily unchanged. The edges of the internal os are drawn upward several centimeters to become a part (both anatomically and functionally) of the lower uterine segment. Effacement may be compared with a funneling process in which the whole length of a narrow cylinder is converted into a very obtuse, flaring funnel with a small circular orifice for an outlet. As the result of increased myometrial activity during uterine preparedness for labor, appreciable effacement of the softened cervix sometimes is accomplished before active labor begins. Effacement causes expulsion of the mucus plug as the cervical canal is shortened.
Compared with the body of the uterus, the lower uterine segment and the cervix are regions of lesser resistance. Therefore, during a contraction, these structures are subjected to distention, in the course of which a centrifugal pull is exerted on the cervix. As the uterine contractions cause pressure on the membranes, the hydrostatic action of the amnionic sac in turn dilates the cervical canal like a wedge. In the absence of intact membranes, the pressure of the presenting part against the cervix and lower uterine segment is similarly effective. Early rupture of the membranes does not retard cervical dilatation so long as the presenting part of the fetus is positioned to exert pressure against the cervix and lower uterine segment. The process of cervical effacement and dilatation causes the formation of the forebag of the amnionic fluid, which is later described in detail.
PATTERN OF CERVICAL DILATATION. Friedman, in his treatise on labor (1978), stated that "the clinical features of uterine contractions—namely, frequency, intensity, and duration—cannot be relied upon as measures of progression in labor nor as indices of normality. ... Except for cervical dilatation and fetal descent, none of the clinical features of the parturient... appears to be useful in assessing labor progression." The pattern of cervical dilatation that takes place during the course of normal labor takes on the shape of a sigmoid curve. As depicted in Figure 11-11, two phases of cervical dilatation are the latent phase and the active phase. The active phase has been subdivided further as the acceleration phase, the phase of maximum slope, and the deceleration phase (Friedman, 1978). The duration of the latent phase is more variable and subject to sensitive changes by extraneous factors and by sedation (prolongation of latent phase) and myometrial stimulation (shortening of latent phase). The duration of the latent phase has little bearing on the subsequent course of labor, whereas the characteristics of the accelerated phase are usually predictive of the outcome of a particular labor. Friedman considers the maximum slope as a "good measure of the overall efficiency of the machine," whereas the nature of the deceleration phase is more reflective of fetopelvic relationships. The completion of cervical dilatation during the active phase of labor is accomplished by cervical retraction about the presenting part of the fetus. After complete cervical dilatation, the second stage of labor commences; thereafter, only progressive descent of the presenting fetal part is available to assess the progress of labor.
PATTERN OF FETAL DESCENT. In many nulliparas, engagement of the fetal head is accomplished before labor begins, and further descent does not occur until late in labor. In others in which engagement of the fetal head is initially not so complete, further descent occurs during the first stage of labor. In the descent pattern of normal labor, a typical hyperbolic curve is formed when the station of the fetal head is plotted as a function of the duration of labor. Active descent usually takes place after cervical dilatation has progressed for some time (Fig. 11-12). In nulliparas, increased rates of descent are observed ordinarily during the phase of maximum slope of cervical dilatation. At this time, the speed of descent increases to a maximum, and this maximal rate of descent is maintained until the presenting fetal part reaches the perineal floor (Friedman, 1978).
CRITERIA FOR NORMAL LABOR. Friedman also sought to select criteria that would delimit normal labor and thereby enable identification of significant abnormalities in labor. The limits, admittedly arbitrary, appear to be logical and clinically useful. The group of women studied were nulliparas and multiparas with no fetopelvic disproportion, no fetal malposition or malpresentation, no multiple pregnancy, and none were treated with heavy sedation or conduction analgesia, oxytocin, or operative intervention. All had a normal pelvis, were at term with a vertex presentation, and delivered average-sized infants. From these studies, Friedman developed the concept of three functional divisions of labor—preparatory, dilatational, and pelvic—to describe the physiological objectives of each division. He found that the preparatory division of labor may be sensitive to sedation and conduction analgesia. Although little cervical dilatation occurs during this time, considerable changes take place in the extracellular matrix (collagen and other connective tissue components) of the cervix. The dilatational division of labor, during which time dilatation is occurring at the most rapid rate, is principally unaffected by sedation or conduction analgesia. The pelvic division of labor begins with the deceleration phase of cervical dilatation. The classical mechanisms of labor, which involve the cardinal movements of the fetus, take place principally during the pelvic division of labor. The onset of the pelvic division is seldom clinically identifiable separate from the dilatational division of labor. Moreover, the rate of cervical dilatation does not always decelerate as full dilatation is approached; in fact, it may accelerate.
RUPTURE OF THE MEMBRANES. Spontaneous rupture of the membranes most often occurs sometime during the course of active labor. Typically, rupture is evident by a sudden gush of a variable quantity of normally clear or slightly turbid, nearly colorless fluid. Less frequently, the membranes remain intact until delivery of the infant. If by chance the membranes remain intact until completion of delivery, the fetus is born surrounded by them, and the portion covering the head of the newborn infant is sometimes referred to as the caul. Rupture of the membranes before the onset of labor at any stage of gestation is referred to as premature rupture of the membranes.
CHANGES IN THE VAGINA AND PELVIC FLOOR.
The birth canal is supported and is functionally closed by a number of layers of tissues that together form the pelvic floor. Its most important structures are the levator ani muscle and the fascia covering its upper and lower surfaces, which for practical purposes may be considered as the pelvic floor (Chap. 3). This group of muscles closes the lower end of the pelvic cavity as a diaphragm and thereby a concave upper and a convex lower surface is presented (see Fig. 3-6). On either side, the levator ani consists of a pubococcygeus and iliococcygeus portion. The posterior and lateral portions of the pelvic floor, which are not filled out by the levator ani, are occupied by the piriformis and coccygeus muscles on either side.
The levator ani varies in thickness from 3 to 5 mm, though its margins encircling the rectum and vagina are somewhat thicker. During pregnancy, the levator ani usually undergoes hypertrophy. By vaginal examination, the internal margin of this muscle can be felt as a thick band that extends backward from the pubis and encircles the vagina about 2 cm above the hymen. On contraction, the levator ani draws both the rectum and vagina forward and upward in the direction of the symphysis pubis and thereby acts to close the vagina. The more superficial muscles of the perineum are too delicate to serve more than an accessory function.
In the first stage of labor, the membranes and presenting part of the fetus serve a role in dilating the upper portion of the vagina. After the membranes have ruptured, however, the changes in the pelvic floor are caused entirely by pressure exerted by the fetal presenting part. The most marked change consists of the stretching of the fibers of the levator ani muscles and the thinning of the central portion of the perineum, which becomes transformed from a wedge-shaped mass of tissue 5 cm in thickness to (in the absence of an episiotomy) a thin, almost transparent membranous structure less than 1 cm in thickness. When the perineum is distended maximally, the anus becomes markedly dilated and presents an opening that varies from 2 to 3 cm in diameter and through which the anterior wall of the rectum bulges. The extraordinary number and size of the blood vessels that supply the vagina and pelvic floor effects a great increase in the amount of blood loss when these tissues are torn.
The third stage of labor begins immediately after delivery of the fetus and involves the separation and expulsion of the placenta. After delivery of the placenta and fetal membranes, active labor is completed. As the baby is born, the uterus spontaneously contracts down on its diminishing contents. Normally, by the time the infant is completely delivered, the uterine cavity is nearly obliterated and the organ consists of an almost solid mass of muscle, several centimeters thick above the thinner lower segment. The uterine fundus now lies just below the level of the umbilicus. This sudden diminution in uterine size is inevitably accompanied by a decrease in the area of the placental implantation site (Fig. 11-13). For the placenta to accommodate itself to this reduced area, it increases in thickness, but because of limited placental elasticity, it is forced to buckle. The resulting tension causes the weakest layer of the decidua—the spongy layer, or decidua spongiosa—to give way, and cleavage takes place at that site. Therefore, separation of the placenta results primarily from a disproportion created between the unchanged size of the placenta and the reduced size of the underlying implantation site. During cesarean delivery, this phenomenon may be directly observed when the placenta is implanted posteriorly.
Cleavage of the placenta is greatly facilitated by the nature of the loose structure of the spongy decidua, which may be likened to the row of perforations between postage stamps. As separation proceeds, a hematoma forms between the separating placenta and the remaining decidua. Formation of the hematoma is usually the result, rather than the cause, of the separation, because in some cases bleeding is negligible. The hematoma may, however, accelerate the process of cleavage. Because the separation of the placenta is through the spongy layer of the decidua, part of the decidua is cast off with the placenta, whereas the rest remains attached to the myometrium. The amount of decidual tissue retained at the placental site varies.
Placental separation ordinarily occurs within a very few minutes after delivery. Brandt (1933) and others, based on results obtained in combined clinical and radiographic studies, supported the idea that because the periphery of the placenta is probably the most adherent portion, separation usually begins elsewhere. Occasionally some degree of separation begins even before the third stage of labor, probably accounting for certain cases of fetal heart rate decelerations that occur just before expulsion of the infant.
SEPARATION OF AMNIOCHORION.
The great decrease in the surface area of the cavity of the uterus simultaneously causes the fetal membranes (amniochorion) and the parietal decidua to be thrown into innumerable folds that increase the thickness of the layer from less than 1 mm to 3 to 4 mm. The lining of the uterus early in the third stage indicates that much of the parietal layer of decidua parietalis is included between the folds of the festooned amnion and chorion laeve (Fig. 11-14). The membranes usually remain in situ until the separation of the placenta is nearly completed. These are then peeled off the uterine wall, partly by the further contraction of the myometrium and partly by traction that is exerted by the separated placenta, which lies in the thin lower uterine segment or in the upper portion of the vagina. The body of the uterus at that time normally forms an almost solid mass of muscle, the anterior and posterior walls of which, each measuring 4 to 5 cm in thickness, lie in close apposition such that the uterine cavity is almost obliterated.
After the placenta has separated from its implantation site, the pressure exerted upon it by the uterine walls causes it to slide downward into the lower uterine segment or the upper part of the vagina. In some cases the placenta may be expelled from those locations by an increase in abdominal pressure, but women in the recumbent position frequently cannot expel the placenta spontaneously. An artificial means of completing the third stage is therefore generally required. The usual method employed is alternately to compress and elevate the fundus, while exerting minimal traction on the umbilical cord .
MECHANISMS OF PLACENTAL EXTRUSION.
When the central, or usual, type of placental separation occurs, the retroplacental hematoma is believed to push the placenta toward the uterine cavity, first the central portion and then the rest. The placenta, thus inverted and weighted with the hematoma, then descends. Because the surrounding membranes are still attached to the decidua, the placenta can descend only by dragging the membranes along; the membranes then peel off its periphery. Consequently, the sac formed by the membranes is inverted, with the glistening amnion over the placental surface presenting at the vulva. The retroplacental hematoma either follows the placenta or is found within the inverted sac. In this process, known as the Schultze mechanism of placental expulsion, blood from the placental site pours into the inverted sac, not escaping externally until after extrusion of the placenta. The other method of placental extrusion is known as the Duncan mechanism, in which separation of the placenta occurs first at the periphery, with the result that blood collects between the membranes and the uterine wall and escapes from the vagina. In this circumstance, the placenta descends to the vagina sideways, and the maternal surface is the first to appear at the vulva.
PHYSIOLOGICAL AND BIOCHEMICAL PROCESSES OF PARTURITION
The physiological processes in human pregnancy that results in the initiation of parturition and the onset of labor are not defined. Until recently, it was generally accepted that successful pregnancy in all mammalian species was dependent upon the action of progesterone to maintain uterine quiescence until near the end of gestation. This assumption was supported by the finding that in the majority of mammalian pregnancies studied, progesterone withdrawal (whether naturally occurring or surgically or pharmacologically induced) precedes the initiation of parturition. In many of these species, a decline, sometimes precipitous, in the levels of progesterone in maternal plasma normally begins after approximately 95 percent of pregnancy. Moreover, the administration of progesterone to these species late in pregnancy delays the onset of parturition.
In primate pregnancy (including humans), however, progesterone withdrawal does not precede the initiation of parturition. The levels of progesterone in the plasma of pregnant women increase throughout pregnancy, declining only after delivery of the placenta, the tissue site of progesterone synthesis in human pregnancy.
Presently, there appear to be two general theorems. Viewed simplistically, these are the retreat from pregnancy maintenance hypothesis and the uterotonin induction of parturition theory. Several combinations of selected tenets of these two postulates are incorporated into the theorems of most investigators. Some researchers also speculate that the mature human fetus, in some undefined fashion, is the source of the initial signal for the commencement of the parturitional process. This has little direct experimental support in human parturition.
Other investigators suggest that one or another uterotonin, produced in increased amounts or in response to an increase in the population of its myometrial receptors, is the proximate cause of the initiation of human parturition. Indeed, an obligatory role for one or more uterotonins is included in most parturition theories, either as a primary or a secondary phenomenon in the final events of childbirth.
UTERINE PHASES OF PARTURITION
Parturition, the bringing forth of young, encompasses all physiological processes involved in birthing: the prelude to, the preparation for, the process of, and the parturient's recovery from childbirth. From the disparate nature of these physiological processes, it is evident that multiple transformations in uterine function must be accommodated in a timely manner during successful pregnancy and parturition. As shown in Figure 11-15, parturition can be arbitrarily divided into four uterine phases which correspond to the major physiological transitions of the myometrium and cervix during pregnancy (Casey and MacDonald 1993a, 1993c; MacDonald and Casey, 1996).
UTERINE PHASE OF PARTURITION.
Beginning even before implantation, a remarkably effective period of myometrial quiescence is imposed on the uterus. This phase of parturition is characterized by myometrial smooth muscle tranquility with maintenance of cervical structural integrity. This is the phase in which the inherent propensity of the myometrium to contract is harnessed. During this phase, which persists for about the first 95 percent of normal pregnancy, myometrial smooth muscle is rendered unresponsive to natural stimuli and relative contractile paralysis is imposed against a host of mechanical and chemical challenges that otherwise would promote emptying of the uterine contents. The myometrial contractile unresponsiveness of phase 0 is so extraordinary that near the end of pregnancy the myometrium must be awakened from this prolonged parturitional diapause in preparation for labor.
During phase 0 of parturition, as the myometrium is maintained in a quiescent state, the cervix must remain firm and unyielding. The maintenance of cervical anatomical and structural integrity is essential to the success of phase 0 of parturition. Premature cervical dilatation, structural incompetence, or both, portend an unfavorable pregnancy outcome that ends most often in preterm delivery (Chap. 27). Shortening of the cervix, when identified between 24 and 28 weeks of pregnancy, is indicative of increased risk of preterm delivery (Iams and colleagues, 199
UTERINE PHASE 1 OF PARTURITION.
To prepare the uterus for labor, the uterine tranquility of phase 0 of parturition must be suspended; this is the time of uterine awakening. The morphological and functional changes in myometrium and cervix that prepare the uterus for labor may be the natural outcome of the suspension of uterine phase 0; but whatever the mechanism, the capacity of myometrial cells to regulate the concentration of cytoplasmic Ca2+ is restored; myometrial cell responsitivity is reinstituted, uterotonin sensitivity develops, and intercellular communicability is established. As these functional capacities of myometrial smooth muscle to contract are implemented and the cervix is ripened, phase 1 of parturition merges into phase 2, active labor. Challis and Lye (1994) refer to the change in uterine functionality before labor as "activation."
UTERINE MODIFICATION DURING PHASE 1 OF PARTURITION. Specific modifications in uterine function evolve with the suspension of uterine phase 0:
1. A striking increase in myometrial oxytocin receptors.
2. An increase in gap junctions (number and surface area) between myometrial cells.
3. Uterine irritability.
4. Responsiveness to uterotonins.
5. Transition from a contractile state characterized predominantly by occasional painless contractions to one in which more frequent contractions develop.
6. Formation of the lower uterine segment.
7. Cervical softening.
With the development of a well-formed lower uterine segment, the fetal head oftentimes descends to or even through the maternal inlet of the pelvis, a distinctive event referred to as lightening. The abdomen of the pregnant woman commonly undergoes a change in shape, an event sometimes described by the mother as "the baby dropped." No doubt there are many other modifications of the uterus late in pregnancy during phase 1, some of which may be integral components of uterine preparedness for labor.
Late in pregnancy, at sometime during phase 1 of parturition, there is a striking—50-fold or more—increase in the number of oxytocin receptors in myometrium (Fuchs and associates, 1982). This coincides with the increase in uterine contractile responsiveness to oxytocin (Soloff and co-workers, 1979). Also, prolonged human gestation is associated with a delay in this increase in receptors (Fuchs and collaborators, 1984).
Also during phase 1, the number and size of gap junctions between myometrial cells increase before the onset of labor, continue to increase during labor, and then decrease quickly after delivery. This is true of spontaneous parturition, both at term and preterm (Garfield and Hayashi, 1981).
CERVICAL CHANGES OF PHASE 1 OF PARTURITION. The body of the uterus (the fundus) and the cervix, although parts of the same organ, must respond in quite different ways during pregnancy and parturition. On the one hand, it is essential that during most of pregnancy, the myometrium be dilatable but remain quiescent. On the other hand, the cervix must remain unyielding and reasonably rigid. Coincident with the initiation of parturition, however, the cervix must soften, yield, and become more readily dilatable. The fundus must be transformed from the relatively relaxed, unresponsive organ characteristic of most of pregnancy to one that will produce effective contractions that drive the fetus through the yielding (dilatable) cervix and on through the birth canal. Failure of a coordinated interaction between the functions of fundus and cervix portends an unfavorable pregnancy outcome. But despite the apparent reversal of roles between cervix and fundus from before to during labor, it is likely that the processes in both portions of the uterus are regulated by common agents.
COMPOSITION OF THE CERVIX. There are three principal structural components of the cervix: collagen, smooth muscle, and the connective tissue or ground substance. Constituents of the cervix important in cervical modifications at parturition are those in the extracellular matrix and ground substance, the glycosaminoglycans, dermatan sulfate and hyaluronic acid. The smooth muscle content of cervix is much less than that of the fundus, and varies anatomically from 25 to only 6 percent.
CERVICAL SOFTENING. The cervical modifications of phase 1 of parturition principally involve changes that occur in collagen, connective tissue, and its ground substance. Cervical softening is associated with two complementary changes:
1. Collagen breakdown and rearrangement of the collagen fibers.
2. Alterations in the relative amounts of the various glycosaminoglycans.
Hyaluronic acid is associated with the capacity of a tissue to retain water. Near term, there is a striking increase in the relative amount of hyaluronic acid in cervix, with a concomitant decrease in dermatan sulfate. The role for smooth muscle in the cervical softening process is not clear, but may be more important than previously believed. Rath and colleagues (1998) and Winkler and Rath (1999) have addressed this possibility in some detail.
UTERINE PHASE 2 OF PARTURITION. Phase 2 is synonymous with active labor, that is, the uterine contractions that bring about progressive cervical dilatation and delivery of the conceptus. Phase 2 of parturition is customarily divided into the three stages of labor described earlier in the chapter. The onset of labor is the transition from uterine phase 1 to phase 2 of parturition.
UTERINE PHASE 3 OF PARTURITION. Phase 3 encompasses the events of the puerperium—maternal recovery from childbirth, maternal contributions to infant survival, and the restoration of fertility in the parturient. Immediately after delivery of the conceptus, and for about an hour or so thereafter, the myometrium must be held in a state of rigid and persistent contraction/retraction, which effects compression of the large uterine vessels and thrombosis of their lumens. In this coordinated fashion, fatal postpartum hemorrhage is prevented.
During the early puerperium, a maternal-type behavior pattern develops and maternal-infant bonding begins. The onset of lactogenesis and milk let-down in maternal mammary glands also is, in an evolutionary sense, crucial to the bringing forth of young. Finally, involution of the uterus, which restores this organ to the nonpregnant state, and the reinstitution of ovulation must be accomplished in preparation for the next pregnancy. Four to six weeks usually are required for complete uterine involution; but the duration of phase 3 of parturition is dependent on the duration of breast feeding. Infertility usually persists so long as breast feeding is continued because of lactation (prolactin)-induced anovulation and amenorrhea (Chap. 58, p. 1548).
Labor is a physiologic process that permits a series of extensive physiologic changes in the mother to allow for the delivery of her fetus through the birth canal.
It is defined as progressive cervical effacement, dilatation, or both, resulting from regular uterine contractions that occur at least every 5 minutes and last 30-60 seconds.
1. Uterine contractions – is a regular contractions of uterine musculature. Typically, contractions occur every 5 to 10 minutes and last for 20-25 seconds in the onset of labor. As labor proceeds, the contractions become more frequent, more intense, and last longer. In the end of labor the contractions occur every 2-3 minutes and last for 50-to 60 seconds. They are characterized be strength, duration, and frequency which are important in generating a normal labor pattern.
2. Bearing-down efforts (or pushing) – is the periodic contractions of diaphragm, pelvic floor muscles and prelum abdominale which are added to the force of uterine contractions. Its voluntary expulsive force.
There are three stages of labor, each of which is considered separately.
The first stage (cervical) is from the onset of true labor to complete dilatation of the cervix.
The second stage (pelvic) starts from complete dilatation of the cervix to the delivery of the baby.
The third stage (placental) starts from the birth of the baby to delivery of the placenta. It is divided into two phases: placental separation and its expulsion.
During the first stage of the labor cervical effacement and dilatation occur.
Labor begins with cervical effacement ! Cervical effacement is the thinning of the cervix.
Although cervical softening and early effacement may occur before labor, during the first stage of labor the entire cervical length is retracted into lower uterine segment as a result of myometrial contractile forces and pressure exerted by either the presenting part of fetal membranes.
The length of the first stage may vary in relation to parity; primiparous patients generally experience a longer first stage than do multiparous patients. The minimal dilatation during the first stage is for primiparous 1-1,2cm/hour and multiparous women: 1,2-1,5 cm/hour. If the progress is slower than this, evaluation for uterine dysfunction, fetal malposition, or cephalopelvic disproportion should be undertaken.
During the first stage, the progress of labor may be measured in terms of cervical effacement, cervical dilatation and descent of the fetal head. Uterine contractions should be monitored every 30 minutes by palpation for their frequency, duration, and intensity. For high-risk pregnancies, uterine contractions should be monitored continuously along with the fetal heart rate.
Vaginal examination should be done sparingly to decrease the risk of an intrauterine infection. Cervical effacement and dilatation, the station and position of the presenting part, the presence of molding or caput in vertex presentation should be recorded. Additional examinations may be performed if the patient reports the urge to push ( to determine if the full dilatation has occurred) or if a significant fetal heart rate deceleration occurs ( to examine for a prolapsed umbilical cord).
The fetal heart rate should be evaluated by either auscultation with a stethoscope, by external monitoring with Doppler equipment. In patients with no significant obstetric risk factors, the fetal heart rate should be auscultated at least every 30 minutes in the first stage of labor and after each uterine contraction in the second stage of the labor.
At the beginning of the second stage, the mother usually has a desire to bear down with each contraction. This abdominal pressure, together with uterine contractile force, combines to expel the fetus. In cephalic presentation, the shape of the fetal head may be altered during labor, making the assessment of descent more difficult. Molding is the alteration of the relationship of the fetal cranial bones to each other as the result of the compressive forces exerted by the bony maternal pelvis.
The second stage generally takes from 30 minutes to 2 hours in primigravid women and from 10-50 minutes in multigravid women. The median duration is 50 minutes in a primipara and slightly under 20 minutes in a multipara.
Clinical management of the second stage of labor. When delivery is imminent, the patient is usually placed in the lithotomy position.
With each contraction, the mother should be encouraged to hold her breath and bear down with expulsive efforts. As the perineum becomes flattened by the crowning head, an episiotomy may be performed, to prevent perineal lacerations.
As the fetal head crowns (i.e., distends the vaginal opening),
PERINEAL PROTECTIVE MANEUVERS
are performed to avoid injury of the fetus and laceration of the perineum:
The first one is prevention of preterm fetal head extension (during pushing efforts the fetal head is flexed).
Second is the delivery of the fetal head out of the pushing by extension of vulvar muscles.
Third one is decreasing of perineal tension by borrowing of the tissues from the upper part of vulva ring to the lower.
Forth is regulations of voluntary maternal effort (pushing) - woman in labor breaths deeply when the fetus is delivered to the level if parietal tubes. At this moment pushing efforts are contraindicated.
Fifth is the delivery of shoulders – first downward, later upward direction of traction are indicated.
The delivery of the placenta occurs during the third stage of labor. Separation of the placenta generally occurs within 2 to 10 minutes of the end of the second stage of labor. Squeezing of the fundus to hasten placental separation is not recommended because it may increase the likelihood of passage of fetal cells into the maternal circulation.
SIGNS OF PLACENTAL SEPARATION ARE FOLLOWS:
Alfeld’s sign – the umbilical cord lengthens outside the vagina, the clamp, applied on an umbilical cord on the level of pudendal cleft, after placental separation comes down on 10-12 cm.
Shreder’s sign – the uterine fundus rises up, the uterus becomes firm and globular.
Krede-Lasarevich’s sign – a doctor presses with his palm above the patient’s pubis. Before placental separation umbilical cord comes inside a vagina (sign is negative), after separation – comes down (sign is positive).
Only when these signs have appeared the attempt to remove of separated placenta should perform. The placenta should be examined to ensure its complete removal and to detect placental abnormalities. If the patient is at risk of postpartum hemorrhage (e.g., because of anemia, prolonged oxytocic augmentation of labor, multiple gestation or hydroamnions), manual removal of the placenta, manual exploration of the uterus, or both may be necessary. After the placental delivery, the cervix and vagina should be thoroughly inspected for lacerations and surgical repair performed if necessary.
LABOR WITH OCCIPUT PRESENTATIONS
The fetus is in the occiput or vertex presentation in approximately 95 percent of all labors. Presentation is most commonly ascertained by abdominal palpation and confirmed by vaginal examination sometime before or at the onset of labor. In the majority of cases, the vertex enters the pelvis with the sagittal suture in the transverse pelvic diameter (Caldwell and associates, 1934). The fetus enters the pelvis in the left occiput transverse (LOT) position in 40 percent of labors, compared with 20 percent in the right occiput transverse (ROT) position (Caldwell and associates, 1934). In occiput anterior positions (LOA1 or ROA), the head either enters the pelvis with the occiput rotated 45 degrees anteriorly from the transverse position, or subsequently does so. The mechanism of labor usually is very similar to that in occiput transverse positions.
In about 20 percent of labors, the fetus enters the pelvis in an occiput posterior (OP) position. The right occiput posterior (ROP2) is slightly more common than the left (LOP) (Caldwell and associates, 1934). It appears likely from evidence obtained by radiographic studies that posterior positions are more often associated with a narrow forepelvis. They are also more commonly seen in association with anterior placentation (Gardberg and Tuppurainen, 1994a).
OCCIPUT ANTERIOR PRESENTATION
Because of the irregular shape of the pelvic canal and the relatively large dimensions of the mature fetal head, it is evident that not all diameters of the head can necessarily pass through all diameters of the pelvis. It follows that a process of adaptation or accommodation of suitable portions of the head to the various segments of the pelvis is required for vaginal delivery. These positional changes in the presenting part constitute the mechanisms of labor.
ENGAGEMENT. As discussed in Chapter 3 (p. 58), the mechanism by which the biparietal diameter, the greatest transverse diameter of the fetal head in occiput presentations, passes through the pelvic inlet is designated engagement. This phenomenon may take place during the last few weeks of pregnancy, or it may not occur until after the commencement of labor. In many multiparous and some nulliparous women, the fetal head is freely movable above the pelvic inlet at the on-set of labor. In this circumstance, the head is some-times referred to as "floating." A normal-sized head usually does not engage with its sagittal suture directed anteroposteriorly. Instead, the fetal head usually enters the pelvic inlet either in the transverse diameter or in one of the oblique diameters (Caldwell and colleagues, 1934).
Occiput presentations occur in about 95% of all labors. Because of the irregular shape of the pelvic canal and the relatively large dimensions of the mature fetal heard, it is evident that not all diameters of the heard can necessarily pass through all diameters of the pelvis. It follows that a process of adaptation or accommodation of suitable portions of the head to the various segments of the pelvis is required for completion of childbirth.
There are 2 kinds of the occiput presentations – anterior and posterior.
The cardinal movements of labor in anterior occiput presentation are:
· internal rotation;
· internal rotation of the fetal head and external rotation of the fetal body.
The various movements are often described as through they occurred separately and independently. In reality, the mechanism of labor consists of a combination of movements that are going on the same time. For example, as part of process of engagement, there is both flexion and descent of the head. It is manifestly impossible for the movements to be completed unless the presenting part descends simultaneously. The uterine contractions effect important modifications in the attitude, or habitus of the fetus especially after the head has descended into the pelvis. These changes consists principally in a straightening of the fetus, with loss of its dorsal convexity and closer application of the extremities and small parts of the body. As a result, the fetal ovoid is transformed into cylinder with normally the smallest possible cross section passing through the birth canal.
Synclitism and asynclitism. Synclitism is a position when the sagittal suture is in the transverse pelvic diameter. The sagittal suture lie exactly midway between the symphysis and promontory.
If the sagittal suture approaches the sacral promontory, more of the anterior parietal bone presents itself to the examining fingers and the condition is called anterior asynclitism. If the sagittal suture lies close to the symphysis more of the posterior parietal bone presents and the condition is called posterior asynclitism.
The cardinal movements of labor in anterior occiput presentation are:
1. Flexion. As soon as descending head meets resistance, whether from the cervix, the walls of the pelvis, or the pelvic floor, flexion of the head are normally results. In this movement, the chin is brought into more intimate contact with the fetal thorax, and the shorter suboccipitobregmatic diameter is substituted for the longer occipito-frontal diameter. The leader point is the area of the small fontanel.
2. Internal rotation. This movement is a manner that the occiput gradually moves from its original position anteriorly towards the symphysis pubis os. The rotation begins when the fetal head descends from the plane of greatest pelvic dimensions to the least pelvic dimensions (midpelvis). The rotation is complete when the head reaches the pelvic floor, the sagittal suture is in the anteroposterior diameter of the pelvic outlet and the small fontanel is under the symphysis.
3. Extension. After internal rotation the sharply flexed head reaches the pelvic floor, two forces come into play. The first, exerted by the uterus, acts more posteriorly, and the second, supplied by the resistant pelvic floor, acts more anteriorly. The resultant force is the direction of the vulvar opening, thereby causing extension. Extension begins when the fixing point (fossa suboccipitalis) is under the inferior margin of the symphysis pubis. With increasing distension of the perineum and vaginal opening, an increasingly large portion of the occiput gradually appears. The head is born by further extension as the occiput, bregma, forehead, nose, mouse.
4. Internal rotation of the fetal head and external rotation of the fetal body. During the head extension the fetal body is in the pelvic cavity. The biacromial diameter turns from the oblique to the anterioposterior diameter of the pelvic outlet. Thus one shoulder is anterior behind the symphysis and the other is posterior. This movement is brought about apparently by the same pelvic factors that effect internal rotation of the head. The anterior shoulder comes under the symphysis pubis, the fetal body flexed and posterior shoulder is born first. Then the anterior shoulder is born. Fetal head rotates as a result of the body rotation. In the I position fetal face turns towards the right, in the II position towards the left. After delivery of the shoulders, the rest of the body of the child is quickly extruded.
The cardinal movements of labor in posterior occiput presentation are:
1. Flexion. The fetal head flexed and presents the suboccipito-frontal (10 cm) diameter in oblique size of the pelvic inlet. The leader point is a middle part of sagittal suture.
2. Internal rotation. The fetal head passes through the pelvic cavity and in narrow plane it begins rotate. In the outlet plane of pelvis (pelvic floor) the sagittal suture became in the direct (anterioposterior) diameter of the pelvic outlet and the small fontanel is under the sacrum os.
3. Additional flexion. After internal rotation the head reaches the pelvic floor. Fetal head fixes with the area of the border of the hair part of head (the first fixing point) under symphysis pubis and flexes. This process leads to delivery of the vertex.
4. Extension. Extension begins when the second fixing point (fossa suboccipitalis) become under the tip of the sacrum. The head is born by further extension.
5. Internal rotation of the fetal head and external rotation of the fetal body. Shoulder enter to the inlet of small pelvis in oblique size and in pelvic cavity perform the internal rotation to 45 °, in the pelvic floor they stand in the direct (anterioposterior) size. The anterior shoulder comes under the margin of symphysis pubis, the fetal body flexed. The posterior shoulder is born first and then the anterior shoulder is born. The head rotation realize as in anterior occiput presentation.
CHANGES IN SHAPE OF THE FETAL HEAD
In vertex presentations, the fetal head undergoes important characteristic changes in shape as the result of the pressures to which it is subjected during labor. In prolonged labors before complete cervical dilatation, the portion of the fetal scalp immediately over the cervical os becomes edematous, forming a swelling known as the caput succedaneum (Fig. 12-20). It usually attains a thickness of only a few millimeters, but in prolonged labors it may be sufficiently extensive to prevent the differentiation of the various sutures and fontanels. More commonly the caput is formed when the head is in the lower portion of the birth canal and frequently only after the resistance of a rigid vaginal outlet is encountered. Because it occurs over the most dependent area of the head, in LOT3 position it is found over the upper and posterior portion of the right parietal bone, and in ROT positions over the corresponding area of the left parietal bone. It follows that after labor the original position may often be ascertained by noting the location of the caput succedaneum.
Molding describes the change in fetal head shape from external compressive forces. Some molding occurs before labor, possibly related to Braxton Hicks contractions. Although taught in previous editions, most studies indicate that there is seldom overlapping of the parietal bones. Instead, a "locking" mechanism at the coronal and lambdoidal connections prevents such overlapping (Carlan and colleagues, 1991). Molding is associated with a shortened suboccipitobregmatic diameter and a lengthening of the mentovertical diameter. These changes are of greatest importance in contracted pelves or asynclitic presentations. In these circumstances, the degree to which the head is capable of molding may make the difference between spontaneous vaginal delivery versus an operative delivery. Some older literature cited severe head molding as a cause for possible cerebral trauma. Because of the multitude of associated factors, for example, prolonged labor with fetal sepsis and acidosis, it is impossible to quantify the effects of molding with any alleged fetal or neonatal neurological sequelae.
CONDUCT OF NORMAL LABOR AND DELIVERY
The ideal conduct of labor and delivery requires two potentially opposing accommodations on the part of obstetrical providers: first, that birthing be recognized as a normal physiological process that most women experience without complications, and second, that intrapartum complications can arise very quickly and unexpectedly. Thus, providers must simultaneously make the woman and her supporters feel comfortable, yet ensure safety for the mother and infant should complications suddenly develop. The American Academy of Pediatrics and the American College of Obstetricians and Gynecologists (1997) have collaborated in the development of Guidelines for Perinatal Care. These provide detailed information on the appropriate content of intrapartum care to include both personnel and facility requirements..
Pregnant women should be urged to report early in labor rather than to procrastinate until delivery is imminent for fear that they might be experiencing false labor. Early admittance to the labor and delivery unit is important; especially so if during antepartum care the woman, her fetus, or both have been identified as being at risk.
IDENTIFICATION OF LABOR. One of the most critical diagnoses in obstetrics is the accurate diagnosis of labor. If labor is falsely diagnosed, inappropriate interventions to augment labor may be made. Conversely, if labor is not diagnosed, the fetus-infant may be damaged by unexpected complications occurring in sites remote from medical personnel and adequate medical facilities. Although the differential diagnosis between false and true labor is difficult at times, it usually can be made on the basis of the contractions.
Contractions of True Labor
• Contractions occur at regular intervals
• Intervals gradually shorten
• Intensity gradually increases
• Discomfort is in the back and abdomen
• Cervix dilates
• Discomfort is not stopped by sedation
Contractions of False Labor
• Contractions occur at irregular intervals
• Intervals remain long
• Intensity remains unchanged
• Discomfort is chiefly in lower abdomen
• Cervix does not dilate
• Discomfort is usually relieved by sedation
In those instances when a diagnosis of labor cannot be established with certainty, it is often wise to observe the woman over a longer period of time. The general condition of mother and fetus should be ascertained accurately by history and physical examination, including blood pressure, temperature, and pulse. The frequency, duration, and intensity of the uterine contractions should be documented, and the time established when they first become uncomfortable. The degree of discomfort that the mother displays is noted. The heart rate, presentation, and size of the fetus should be determined and documented on admission. The fetal heart rate should be checked, especially at the end of a contraction and immediately thereafter, to identify pathological slowing of the heart rate (Chap. 14, p. 354). Inquiries are made about the status of the fetal membranes and whether there has been any vaginal bleeding. The questions of whether fluid has leaked from the vagina and, if so, how much and when the leakage first commenced are also addressed.
FEDERAL REQUIREMENTS FOR INTER-HOSPITAL TRANSFER OF LABORING WOMEN. All Medicare-participating hospitals with emergency services must provide an appropriate medical screening examination for any pregnant women experiencing contractions who comes to the emergency department for evaluation. The definition of an emergency condition makes specific reference to a pregnant woman who is having contractions. Labor is defined as "...the process of childbirth beginning with the latent phase of labor continuing through delivery of the placenta. A woman experiencing contractions is in true labor unless a physician certifies that after a reasonable time of observation the woman is in false labor." A woman in true labor is considered "unstable" for inter-hospital transfer purposes until the child and placenta are delivered. An unstable woman may, however, be transferred at the direction of the patient or when a physician signs a written certification that benefits of treatment at another facility outweigh the risks of transfer. Physicians and hospitals violating these federal requirements are subject to civil penalties of up to $50,000, as well as termination from the Medicare program.
ELECTRONIC ADMISSION TESTING. Some investigators recommend that a nonstress test (NST) or contraction stress test (CST) be performed on all patients admitted to the labor and delivery unit, the so-called "fetal admission test" (Ingemarsson and associates, 1986). Such fetal surveillance is in reality an assessment of fetal heart rate accelerations or lack of the same with fetal movement (NST1); or an assessment of fetal heart rate before, during, and following a uterine contraction if the patient is in labor (CST2) (Freeman and colleagues, 1991). Fetal heart rate variability and variable decelerations also are used in these evaluations. It has been suggested that such tests of fetal well-being, alone or in combination with fetal acoustic stimulation, will identify unsuspected cases of fetal jeopardy (Ingemarsson and associates, 1988; Sarno and co-workers, 1990). Certainly, if the woman is to be discharged from the labor unit undelivered, this practice is reasonable to ensure, as nearly as possible, that fetal compromise is not identified at this time. At Parkland Hospital, external electronic monitoring is performed for at least one hour before discharging women with false labor.
VAGINAL EXAMINATION. Most often, unless there has been bleeding in excess of bloody show, a vaginal examination under aseptic conditions is performed. Careful attention to the following items is essential in order to obtain the greatest amount of information and to minimize bacterial contamination from multiple examinations.
1. Amnionic fluid. If there is a question of membrane rupture, a sterile speculum is carefully inserted, and fluid is sought in the posterior vaginal fornix. Any fluid is observed for vernix or meconium; if the source of the fluid remains in doubt, it is collected on a swab for further study, as described later.
2. Cervix. Softness, degree of effacement (length), extent of dilatation, and location of the cervix with respect to the presenting part and vagina are ascertained, as will be described. The presence of membranes with or without amnionic fluid below the presenting part often can be felt by careful palpation. The fetal membranes often can be visualized if they are intact and the cervix is dilated somewhat.
3. Presenting part. The nature of the presenting part should be positively determined and, ideally, its position as well, as described in Chapter 12.
4. Station. The degree of descent of the presenting part into the birth canal is identified, as will be described. If the fetal head is high in the pelvis (above the level of the ischial spines), the effect of firm fundal pressure on descent of the fetal head is tested.
5. Pelvic architecture. The diagonal conjugate, ischial spines, pelvic sidewalls, and sacrum are reevaluated for adequacy.
The degree of cervical effacement is usually expressed in terms of the length of the cervical canal compared to that of an uneffaced cervix. When the length of the cervix is reduced by one half, it is 50 percent effaced; when the cervix becomes as thin as the adjacent lower uterine segment, it is completely, or 100 percent, effaced.
This is ascertained by estimating the average diameter of the cervical opening. The examining finger is swept from the margin of the cervix on one side to the opposite side, and the diameter traversed is expressed in centimeters. The cervix is said to be dilated fully when the diameter measures 10 cm, because the presenting part of a term-size infant usually can pass through a cervix this widely dilated.
POSITION OF THE CERVIX.
The relationship of the cervical os to the fetal head is categorized as posterior, midposition, or anterior. A posterior position is suggestive of preterm labor.
The level of the presenting fetal part in the birth canal is described in relationship to the ischial spines, which are halfway between the pelvic inlet and the pelvic outlet. When the lowermost portion of the presenting fetal part is at the level of the ischial spines, it is designated as being at zero (0) station. In the past, the long axis of the birth canal above the ischial spines was arbitrarily divided into thirds. In 1988, the American College of Obstetricians and Gynecologists began using a classification of station that divides the pelvis above and below the spines into fifths. These divisions represent centimeters above and below the spines. Thus, as the presenting fetal part descends from the inlet toward the ischial spines, the designation is -5, -4, -3, -2, -1, then 0 station. Below the ischial spines, the presenting fetal part passes +1, +2, +3, +4, and +5 stations to delivery. Station +5 cm corresponds to the fetal head being visible at the introitus. An approximate correlation of the two methods of describing station is: +2 cm = +1/3 and +4 cm = +2/3 (American Academy of Pediatrics and the American College of Obstetricians and Gynecologists, 1997).
If the leading part of the fetal head is at 0 station or below, most often engagement of the head has occurred; that is, the biparietal plane of the fetal head has passed through the pelvic inlet. If the head is unusually molded, or if there is an extensive caput formation, or both, engagement might not have taken place even though the head appears to be at 0 station.
DETECTION OF RUPTURED MEMBRANES.
The pregnant woman should be instructed during the antepartum period to be aware of leakage of fluid from the vagina and to report such an occurrence promptly. Rupture of the membranes is significant for three reasons. First, if the presenting part is not fixed in the pelvis, the possibility of prolapse of the umbilical cord and cord compression is greatly increased. Second, labor is likely to occur soon if the pregnancy is at or near term. Third, if delivery is delayed for 24 hours or more after membrane rupture, there is increasing likelihood of serious intrauterine infection.
A conclusive diagnosis of rupture of the membranes is made when amnionic fluid is seen pooling in the posterior fornix or clear fluid is passing from the cervical canal (American College of Obstetricians and Gynecologists, 2000). Although several diagnostic tests for the detection of ruptured membranes have been recommended, none is completely reliable. If the diagnosis remains uncertain, another method involves testing the pH of the vaginal fluid; the pH of vaginal secretions normally ranges between 4.5 and 5.5, whereas that of amnionic fluid is usually 7.0 to 7.5. The use of the indicator nitrazine for the diagnosis of ruptured membranes, first suggested by Baptisti (1938), is a simple and fairly reliable method. Test papers are impregnated with the dye, and the color of the reaction is interpreted by comparison with a standard color chart. The pH of the vaginal secretion is estimated by inserting a sterile cotton-tipped applicator deeply into the vagina, and then touching it to a strip of the nitrazine paper and comparing the color of the paper with the chart supplied with the paper. A pH above 6.5 is consistent with ruptured membranes. False-positive tests occur with blood, semen, or bacterial vaginosis and false-negative tests with minimal fluid (American College of Obstetricians and Gynecologists, 2000).
Other tests have been used as markers for rupture of the membranes. Arborization or ferning of vaginal fluid suggests amnionic rather than cervical fluid. Detection of alpha-fetoprotein in the vaginal vault has been used to identify amnionic fluid (Yamada and colleagues, 1998). Unequivocal identification comes from injection of various dyes, including Evans blue, methylene blue, indigo carmine, or fluorescein, into the amnionic sac via abdominal amniocentesis.
VITAL SIGNS AND REVIEW OF PREGNANCY RECORD.
The maternal blood pressure, temperature, pulse, and respiratory rate are checked for any abnormality, and these are recorded. The pregnancy record is promptly reviewed to identify complications. Any problems identified during the antepartum period, as well as any that were anticipated, should be displayed prominently in the pregnancy record.
PREPARATION OF VULVA AND PERINEUM.
The woman is positioned to allow inspection and cleansing of the vulva and perineum. Scrubbing is directed from above, downward, and away from the introitus. Attention should be paid to careful cleansing of the vulvar folds. As the scrub sponge passes over the anal region, it is discarded. If hair on the lower half of the vulva or perineum is felt to interfere at the time of delivery, it can be clipped with scissors or a mini-shave prep can be performed. Routine shaving of the perineum is not performed at Parkland Hospital.
Ideally, after the vulvar and perineal regions have been properly prepared, and the examiner has donned sterile gloves, the thumb and forefinger of one hand are used to separate the labia widely to expose the vaginal opening and prevent the examining fingers from coming in contact with the inner surfaces of the labia. The index and second fingers of the other hand are then introduced into the vagina (Fig. 13-1). A precise routine of evaluation, as described earlier should be followed. It is important to avoid the anal region and not to withdraw the fingers from the vagina until the examination is completed. The number of vaginal examinations during labor does correlate with infectious morbidity, especially in cases of early membrane rupture.
Early in labor, a cleansing enema often is given to minimize subsequent contamination by feces, which otherwise may be a problem during the second stage of labor and delivery. A ready-to-use enema solution of sodium phosphate in a disposable container (Fleet enema) has proven satisfactory. Enemas are not routinely used at Parkland Hospital.
When the woman is admitted in labor, most often the hematocrit, or hemoglobin concentration, should be rechecked. The hematocrit can be measured easily and quickly. Blood may be collected in a plain tube from which a heparinized capillary tube is filled immediately. By employing a small microhematocrit centrifuge in the labor-delivery unit, the value can be obtained in 3 minutes. A labeled tube of blood is allowed to clot and is kept on hand for blood type and screen, if needed, and another is used for routine serology. In some units, a voided urine specimen, as free as possible of debris, is examined for protein and glucose. We obtain a urine specimen for protein analysis only in hypertensive women. Patients who have had no prenatal care should be considered to be at risk for syphilis, hepatitis B, and human immunodeficiency virus (American Academy of Pediatrics and the American College of Obstetricians and Gynecologists, 1997). In unregistered patients, these laboratory studies as well as a blood type, Rh, and antibody screen for atypical antibodies should be performed. Some states, for example Texas, now require routine testing for syphilis, hepatitis B, and human immunodeficiency virus in all women admitted to labor and delivery units.
MANAGEMENT OF FIRST STAGE OF LABOR
As soon as possible after admittance, the remainder of the general physical examination is completed. The physician can best reach a conclusion about the normalcy of the pregnancy when all examinations, including record and laboratory review, are completed. A rational plan for monitoring labor then can be established based on the needs of the fetus and the mother. If no abnormality is identified or suspected, the mother should be reassured. Although the average duration of the first stage of labor in nulliparous women is about 7 hours and in parous women about 4 hours, there are marked individual variations. Any precise statement as to the duration of labor, therefore, is unwise .
MONITORING FETAL WELL-BEING DURING LABOR
It is mandatory for optimal pregnancy outcome that a well-defined program be established that provides careful surveillance of the well-being of both mother and fetus during labor. All observations must be appropriately recorded. The frequency, intensity, and duration of uterine contractions, and the response of the fetal heart rate to the contractions, are of considerable concern. These features can be promptly evaluated in logical sequence.
FETAL HEART RATE.
The fetal heart rate may be identified with a suitable stethoscope or any of a variety of Doppler ultrasonic devices. Changes in the fetal heart rate that most likely are ominous almost always are detectable immediately after a uterine contraction. Therefore, it is imperative that the fetal heart be monitored by auscultation immediately after a contraction. To avoid confusing maternal and fetal heart rates, the maternal pulse should be counted as the fetal heart rate is counted. Otherwise, maternal tachycardia may be misinterpreted as a normal fetal heart rate.
Fetal jeopardy, compromise, or distress—that is, loss of fetal well-being—is suspected if the fetal heart rate immediately after a contraction is repeatedly below 110 bpm. Fetal jeopardy very likely exists if the rate is heard to be less than 100 per minute, even though there is recovery to a rate in the 110 to 160 bpm range before the next contraction. When decelerations of this magnitude are found after a contraction, further labor, if allowed, is best monitored electronically.
The American Academy of Pediatrics and American College of Obstetricians and Gynecologists (1997) recommend that during the first stage of labor, in the absence of any abnormalities, the fetal heart should be checked immediately after a contraction at least every 30 minutes and then every 15 minutes during the second stage. If continuous electronic monitoring is used, the tracing is evaluated at least every 30 minutes during the first stage and at least every 15 minutes during second-stage labor. For women with pregnancies at risk, auscultation is performed at least every 15 minutes during the first stage of labor and every 5 minutes during the second stage. Continuous electronic monitoring may be used with evaluation of the tracing every 15 minutes during the first stage of labor, and every 5 minutes during the second stage.
With the palm of the hand lightly on the uterus, the examiner determines the time of onset of the contraction. The intensity of the contraction is gauged from the degree of firmness the uterus achieves. At the acme of effective contractions, the finger or thumb cannot readily indent the uterus. Next, the time that the contraction disappears is noted. This sequence is repeated in order to evaluate the frequency, duration, and intensity of uterine contractions. It is best to quantify the contractions as regards the degree of firmness or resistance to indentation.
MATERNAL MONITORING AND MANAGEMENT DURING LABOR
MATERNAL VITAL SIGNS. Maternal temperature, pulse, and blood pressure are evaluated at least every 4 hours (Table 13-3). If fetal membranes have been ruptured for many hours before the onset of labor, or if there is a borderline temperature elevation, the temperature is checked hourly. Moreover, with prolonged membrane rupture—defined as greater than 18 hours—antimicrobial administration for prevention of group B streptococcal infections is recommended (American College of Obstetricians and Gynecologists, 1996).
SUBSEQUENT VAGINAL EXAMINATIONS. During the first stage of labor, the need for subsequent vaginal examinations to identify the status of the cervix and the station and position of the presenting part will vary considerably (Table 13-3). When the membranes rupture, an examination should be repeated expeditiously if the fetal head was not definitely engaged at the previous vaginal examination. The fetal heart rate should be checked immediately and during the next uterine contraction in order to detect an occult umbilical cord compression. At Parkland Hospital, periodic pelvic examinations are often performed at 2- to 3-hour intervals to evaluate the progress of labor (Chap. 18, p. 446).
ORAL INTAKE. Food should be withheld during active labor and delivery. Gastric emptying time is remarkably prolonged once labor is established and analgesics are administered. As a consequence, ingested food and most medications remain in the stomach and are not absorbed; instead, they may be vomited and aspirated (Chap. 15, p. 366). There is a trend toward giving liquids in moderation to laboring women (Table 13-3). Guyton and Gibbs (1994) cite studies in which 150 mL of fluids were given orally 2 hours before elective surgery. The incidence of aspiration was not affected. It is unclear whether these studies can be applied to women in labor, who are at risk for urgent cesarean delivery at all times.
INTRAVENOUS FLUIDS. Although it has become customary in many hospitals to establish an intravenous infusion system routinely early in labor, there is seldom any real need for such in the normally pregnant woman at least until analgesia is administered. An intravenous infusion system is advantageous during the immediate puerperium in order to administer oxytocin prophylactically, and at times therapeutically when uterine atony persists. Moreover, with longer labors, the administration of glucose, sodium, and water to the otherwise fasting woman at the rate of 60 to 120 mL/hr is efficacious to prevent dehydration and acidosis
MATERNAL POSITION DURING LABOR. The normal laboring woman need not be confined to bed early in labor. A comfortable chair may be beneficial psychologically and perhaps physiologically. In bed, the laboring woman should be allowed to assume the position she finds most comfortable, which will be lateral recumbency most of the time. She must not be restricted to lying supine. Bloom and colleagues (1998) conducted a randomized trial of walking during labor in over 1000 women with low-risk pregnancies. They found that walking neither enhanced nor impaired active labor and that it was not harmful.
ANALGESIA. Most often, analgesia is initiated on the basis of maternal discomfort. The kinds of analgesia, amounts, and frequency of administration should be based on the need to allay pain on the one hand and the likelihood of delivering a depressed infant on the other.
The timing, method of administration, and size of initial and subsequent doses of systemically acting analgesic agents are based to a considerable degree on the anticipated interval of time until delivery. A repeat vaginal examination is often appropriate before administering more analgesia. With the onset of symptoms characteristic of the second stage of labor, that is, an urge to bear down or "push," the status of the cervix and the presenting part should be reevaluated.
AMNIOTOMY. If the membranes are intact, there is a great temptation even during normal labor to perform amniotomy. The presumed benefits are more rapid labor, earlier detection of instances of meconium staining of amnionic fluid, and the opportunity to apply an electrode to the fetus and insert a pressure catheter into the uterine cavity. The advantages and disadvantages of amniotomy are discussed in Chapter 18 (p. 446). If amniotomy is performed, an aseptic technique should be used. Importantly, the fetal head must be well applied to the cervix and not be dislodged from the pelvis during the procedure; such an action invites prolapse of the umbilical cord.
URINARY BLADDER FUNCTION. Bladder distention should be avoided, because it can lead to obstructed labor and to subsequent bladder hypotonia and infection. During each abdominal examination, the suprapubic region should be visualized and palpated in order to detect a filling bladder. If the bladder is readily seen or palpated above the symphysis, the woman should be encouraged to void. At times she can ambulate with assistance to a toilet and successfully void, even though she could not void on a bedpan. If the bladder is distended and she cannot void, intermittent catheterization is indicated.
MANAGEMENT OF SECOND-STAGE LABOR
With full dilatation of the cervix, which signifies the onset of the second stage of labor, the woman typically begins to bear down, and with descent of the presenting part she develops the urge to defecate. Uterine contractions and the accompanying expulsive forces may last 11/2 minutes and recur at times after a myometrial resting phase of no more than a minute.
The median duration of the second stage is 50 minutes in nulliparas and 20 minutes in multiparas, but it can be highly variable. In a woman of higher parity with a relaxed vagina and perineum, two or three expulsive efforts after the cervix is fully dilated may suffice to complete the delivery of the infant. Conversely, in a woman with a contracted pelvis or a large fetus, or with impaired expulsive efforts from conduction analgesia or intense sedation, the second stage may become abnormally long.
FETAL HEART RATE.
For the low-risk fetus, the heart rate should be auscultated during the second stage of labor at least every 15 minutes, whereas in those at high risk, 5-minute intervals are recommended (American Academy of Pediatrics and the American College of Obstetricians and Gynecologists, 1997). Slowing of the fetal heart rate induced by head compression is common during a contraction and the accompanying maternal expulsive efforts. If recovery of the fetal heart rate is prompt after the contraction and expulsive efforts cease, labor is allowed to continue. Not all instances of fetal heart rate slowing during second-stage labor are the consequence of head compression. The vigorous force generated within the uterus by its contraction and by maternal expulsive efforts may reduce placental perfusion appreciably. Descent of the fetus through the birth canal and the consequent reduction in uterine volume may trigger some degree of premature separation of the placenta, with further compromise of fetal well-being. Descent is more likely to tighten a loop or loops of umbilical cord around the fetus, especially the neck, sufficiently to obstruct umbilical blood flow. Prolonged, uninterrupted maternal expulsive efforts can be dangerous to the fetus in these circumstances. Maternal tachycardia, which is common during the second stage, must not be mistaken for a normal fetal heart rate.
MATERNAL EXPULSIVE EFFORTS.
In most cases, bearing down is reflex and spontaneous during second-stage labor, but occasionally the woman does not employ her expulsive forces to good advantage and coaching is desirable. Her legs should be half-flexed so that she can push with them against the mattress. Instructions should be to take a deep breath as soon as the next uterine contraction begins, and with her breath held, to exert downward pressure exactly as though she were straining at stool. She should not be encouraged to "push" beyond the time of completion of each uterine contraction. Instead, she and her fetus should be allowed to rest and recover from the combined effects of the uterine contraction, breath holding, and considerable physical effort. Gardosi and associates (1989) have recommended a squatting or semi-squatting position using a specialized pillow. They claim that this shortens second-stage labor by increasing expulsive forces and by increasing the diameter of the pelvic outlet. Eason and colleagues (2000) performed an extensive review of positions and their effect on the incidence of perineal trauma. They found that the supported upright position had no advantages over the recumbent one.
Usually, bearing down efforts result in increasing bulging of the perineum—that is, further descent of the fetal head. The woman should be informed of such progress, for encouragement is very important. During this period of active bearing down, the fetal heart rate auscultated immediately after the contraction is likely to be slow, but should recover to normal range before the next expulsive effort.
As the head descends through the pelvis, feces is frequently expelled by the woman. As the head descends still farther, the perineum begins to bulge and the overlying skin becomes tense and glistening. Now the scalp of the fetus may be visible through the vulvar opening (Fig. 13-2). At this time, or before in instances where little perineal resistance to expulsion is anticipated, the woman and her fetus are prepared for delivery.
PREPARATION FOR DELIVERY.
Delivery can be accomplished with the mother in a variety of positions. The most widely used and often the most satisfactory one is the dorsal lithotomy position in order to increase the diameter of the pelvic outlet. In many birthing rooms this is accomplished with the woman lying flat on the bed. For better exposure, leg holders or stirrups are used. In placing the legs in leg holders, care should be taken not to separate the legs too widely or place one leg higher than the other, as this will exert pulling forces on the perineum that might easily result in the extension of a spontaneous tear or an episiotomy into a fourth-degree tear. The popliteal region should rest comfortably in the proximal portion and the heel in the distal portion of the leg-holder. The leg should not be forced to conform to the preexisting setting. The legs are not strapped into the stirrups, thereby allowing quick flexion of the thighs back onto the abdomen should shoulder dystocia be encountered. Cramps in the legs may develop during the second stage in part because of pressure by the fetal head on nerves in the pelvis. Such cramps may be relieved by changing the position of the leg or by brief massage, but leg cramps should never be ignored.
Preparation for delivery entails vulvar and perineal cleansing. If desired, sterile drapes may be placed in such a way that only the immediate area about the vulva is exposed (Fig. 13-3). In the past, the major reason for care in scrubbing, gowning, and gloving was to protect the laboring woman from the introduction of infectious agents. Although these considerations remain valid, concern today also must be extended to the health-care providers, because of the threat of exposure to human immunodeficiency virus. Recommendations for protection of those who care for women during labor and delivery are summarized in Chapter 57 (p. 1498).
DELIVERY OF THE HEAD
With each contraction, the perineum bulges increasingly and the vulvovaginal opening becomes more dilated by the fetal head ), gradually forming an ovoid and finally an almost circular opening. With the cessation of each contraction, the opening becomes smaller as the head recedes. As the head becomes increasingly visible, the vaginal outlet and vulva are stretched further until they ultimately encircle the largest diameter of the fetal head (Fig. 13-5). This encirclement of the largest head diameter by the vulvar ring is known as crowning.
Unless an episiotomy has been made, as described later in the chapter, the perineum by now is extremely thin and, especially in the case of the nulliparous woman, may undergo spontaneous laceration. At the same time, the anus becomes greatly stretched and protuberant, and the anterior wall of the rectum may be easily seen through it. Over many years there has been considerable controversy concerning whether an episiotomy should be cut. We advocate individualization and do not routinely cut an episiotomy. It is now clear that an episiotomy will increase the risk of a tear into the external anal sphincter and/or the rectum. Conversely, anterior tears involving the urethra and labia are much more common in women in whom an episiotomy is not cut.
Immediately after delivery of the infant, there is usually a gush of amnionic fluid, often tinged with blood but not grossly bloody.
CLEARING THE NASOPHARYNX. To minimize the likelihood of aspiration of amnionic fluid, debris, and blood that might occur once the thorax is delivered and the infant can inspire, the face is quickly wiped and the nares and mouth are aspirated.
NUCHAL CORD. Following delivery of the anterior shoulder, the finger should be passed to the neck of the fetus to ascertain whether it is encircled by one or more coils of the umbilical cord (Fig. 13-11). Nuchal cords occur in about 25 percent of cases and ordinarily do no harm. If a coil of umbilical cord is felt, it should be drawn down between the fingers and, if loose enough, slipped over the infant's head. If it is applied too tightly to the neck to be slipped over the head, it should be cut between two clamps and the infant promptly delivered.
CLAMPING THE CORD
The umbilical cord is cut between two clamps placed 4 or 5 cm from the fetal abdomen, and later an umbilical cord clamp is applied 2 or 3 cm from the fetal abdomen. A plastic clamp (Hollister, Double Grip Umbilical Clamp) that is safe, efficient, easy to sterilize, and fairly inexpensive is shown in Figure 13-12.
TIMING OF CORD CLAMPING.
If, after delivery, the infant is placed at or below the level of the vaginal introitus for 3 minutes and the fetoplacental circulation is not immediately occluded by clamping the cord, an average of 80 mL of blood may be shifted from the placenta to the infant (Yao and Lind, 1974). One benefit to be derived from placental transfusion is that the hemoglobin in 80 mL of placental blood that shifts to the fetus eventually provides about 50 mg of iron, which reduces the frequency of iron-deficiency anemia later in infancy. In the presence of accelerated destruction of erythrocytes, as occurs with maternal alloimmunization, the bilirubin formed from the added erythrocytes contributes further to the danger of hyperbilirubinemia (Chap. 39, p. 1061). Although the theoretical risk of circulatory overloading from gross hypervolemia is formidable, especially in preterm and growth-retarded infants, addition of placental blood to the otherwise normal infant's circulation ordinarily does not cause difficulty.
Our policy is to clamp the cord after first thoroughly clearing the airway, all of which usually takes about 30 seconds. The infant is not elevated above the introitus at vaginal delivery or much above the maternal abdominal wall at the time of cesarean delivery.
MANAGEMENT OF THE THIRD STAGE
Immediately after delivery of the infant, the height of the uterine fundus and its consistency are ascertained. As long as the uterus remains firm and there is no unusual bleeding, watchful waiting until the placenta is separated is the usual practice. No massage is practiced; the hand is simply rested on the fundus frequently, to make certain that the organ does not become atonic and filled with blood behind a separated placenta.
SIGNS OF PLACENTAL SEPARATION
Because attempts to express the placenta prior to its separation are futile and possibly dangerous, it is most important that the following signs of placental separation be recognized:
1. The uterus becomes globular and, as a rule, firmer. This sign is the earliest to appear.
2. There is often a sudden gush of blood.
3. The uterus rises in the abdomen because the placenta, having separated, passes down into the lower uterine segment and vagina, where its bulk pushes the uterus upward.
4. The umbilical cord protrudes farther out of the vagina, indicating that the placenta has descended.
These signs sometimes appear within about 1 minute after delivery of the infant and usually within 5 minutes. When the placenta has separated, it should be ascertained that the uterus is firmly contracted. The mother may be asked to bear down, and the intra-abdominal pressure so produced may be adequate to expel the placenta. If these efforts fail, or if spontaneous expulsion is not possible because of anesthesia, and after ensuring that the uterus is contracted firmly, pressure is exerted with the hand on the fundus to propel the detached placenta into the vagina, as depicted and described in Figure 13-13. This approach has been termed physiological management, as later to be contrasted with "active management" of the third stage (Thilaganathan and colleagues, 1993).
DELIVERY OF THE PLACENTA
Placental expression should never be forced before placental separation lest the uterus be turned inside out. Traction on the umbilical cord must not be used to pull the placenta out of the uterus. Inversion of the uterus is one of the grave complications associated with delivery (Chap. 25, p. 642). As pressure is applied to the body of the uterus (Fig. 13-13), the umbilical cord is kept slightly taut. The uterus is lifted cephalad with the abdominal hand. This maneuver is repeated until the placenta reaches the introitus (Prendiville and associates, 1988b). As the placenta passes through the introitus, pressure on the uterus is stopped. The placenta is then gently lifted away from the introitus (Fig. 13-14). Care is taken to prevent the membranes from being torn off and left behind. If the membranes start to tear, they are grasped with a clamp and removed by gentle traction (Fig. 13-15). The maternal surface of the placenta should be examined carefully to ensure that no placental fragments are left in the uterus.
MANUAL REMOVAL OF PLACENTA.
Occasionally, the placenta will not separate promptly. This is especially common in cases of preterm delivery (Dombrowski and colleagues, 1995).. It is unclear as to the length of time that should elapse in the absence of bleeding before the placenta is manually removed. Manual removal of the placenta is rightfully practiced much sooner and more often than in the past. In fact, some obstetricians practice routine manual removal of any placenta that has not separated spontaneously by the time they have completed delivery of the infant and care of the cord in women with conduction analgesia. Proof of the benefits of this practice, however, has not been established, and most obstetricians await spontaneous placental separation unless bleeding is excessive.
ACTIVE MANAGEMENT OF THE THIRD STAGE
Thilaganathan and associates (1993) compared a regimen of active management with syntometrine (5 units of oxytocin with 0.5 mg of ergometrine) and controlled cord traction with one of physiological management wherein the cord was not clamped and the placenta was delivered by maternal efforts. Among 103 low-risk term deliveries, active management resulted in a reduction in the length of the third stage of labor, but no reduction in blood loss compared with physiological management. Mitchell and Elbourne (1993) found that syntometrine administered intramuscularly concurrent with delivery of the anterior shoulder was more effective than oxytocin (5 units intramuscularly) alone in the prevention of postpartum hemorrhage. Duration of the third stage of labor and need for manual removal of the placenta were similar. Side effects of nausea, vomiting, and blood pressure elevations with ergometrine prevented any recommendation for its routine usage.
Puerperium is strictly defined as the period of confinement during and just after birth. By popular use, however, the meaning usually includes the 6 subsequent weeks during which normal pregnancy involution occurs (Hughes, 1972). Of course, and as described in Chapter 8, maternal adaptations to pregnancy do not necessarily all subside completely by 6 weeks postpartum.
CLINICAL AND PHYSIOLOGICAL ASPECTS OF THE PUERPERIUM
The pueperium consists of the period following delivery of the baby and placenta to approximately 6 weeks postpartum. During the puerperium, the reproductive organs and maternal physiology return toward the pregnancy state although menses may not return for much longer.
Involution of the uterus. Immediate after delivery, the fundus of the uterus is easily palpable on the level of the umbilicus. The immediate reduction in uterine size is the result of delivery of the fetus, placenta and amniotic fluid as well as the loss of hormonal stimulation. Further uterine involution is caused by autolysis of intracellular myometrial protein, resulting in a decrease in cell size but not cell number. Through these changes, the uterus returns.
As the myometrial fibers contract, the blood clots from uterus are expelled and the thrombi in the large vessels of the placental bed undergo organization. Within the first 3 days, the remaining decidua differentiates into a superficial layer, which becomes necrotic and sloughs, and a basal layer adjacent to the myometrium, which contained the fundi of the endometrial glands and is the source of the new endometrium.
Immediately after the delivery of the placenta, the uterus is palpated bimanually to ascertain that it is firm.
This discharge is fairly heavy at first and rapidly decreases in amount over the first 2 to 3 days postpartum, although it may last for several weeks.
1-3 day after labor – bloody
4-6 day after labor – bloody-serous
7-9 day after labor – serous-bloody
10 day after labor – serous
For the first few days after delivery, the uterine discharge appears red ( lochia rubra) owing the presents of erythrocytes. After 3 to 4 days, the lochia becomes paler ( lochia serosa), and by the tenth day, it assumes a white or yellow- white color ( lochia alba). By the end of the third week postpartum, the endometrium is reestablished in most patients.
Cervix. Just after the labor the cervix admits the hand. Within several hours of delivery the cervix has reformed, and on 4-5 day it usually admits only one finger (i.e., it is approximately 1cm in diameter), on 9-10 day cervix closed. The round shape of the nulliparous cervix is usually permanently replaced by a transverse, fish-mouth shaped external os, the result of laceration during delivery. Vulvar and vaginal tissues return to normal over the first several days, although the vaginal mucosa reflects a hypoestrogenic state if the woman breast-feeds because ovarian function is suppressed during breast-feeding.
Abdominal wall. Return of the elastic fibers of the stretched rectus muscles to normal configuration occurs slowly and is aided by exercise.
At time of delivery, the drop of estrogen and other placental hormones is a major factor in removing the inhibition of the action of prolactin. also, suckling by the infant stimulates release of oxytocin from the neurohypophysis. On approximately the second day after delivery, colostrum is secreted. After about 3 to 6 days, the colostrum is replaced by mature milk.
CHANGES IN THE CERVIX AND LOWER UTERINE SEGMENT.
The outer cervical margin, which corresponds to the external os, is usually lacerated, especially laterally. The cervical opening contracts slowly, and for a few days immediately after labor it readily admits two fingers. By the end of the first week, it has narrowed. As the opening narrows, the cervix thickens, and a canal reforms. At the completion of involution, however, the external os does not resume its pregravid appearance completely. It remains somewhat wider, and typically, bilateral depressions at the site of lacerations remain as permanent changes that characterize the parous cervix. It should also be kept in mind that the cervical epithelium undergoes considerable remodeling as a result of childbirth. For example, Ahdoot and colleagues (1998) found that approximately 50 percent of women with high-grade squamous intraepithelial cells showed regression as a result of vaginal delivery.
The markedly thinned-out lower uterine segment contracts and retracts but not as forcefully as the body of the uterus. Over the course of a few weeks, the lower segment is converted from a clearly evident structure, large enough to contain most of the fetal head, into a barely discernible uterine isthmus located between the uterine corpus above and the internal cervical os below .
INVOLUTION OF THE UTERINE CORPUS.
Immediately after placental expulsion, the fundus of the contracted uterus is slightly below the umbilicus. The uterine body then consists mostly of myometrium covered by serosa and lined by basal decidua. The anterior and posterior walls, in close apposition, each measure 4 to 5 cm in thickness. Because its vessels are compressed by the contracted myometrium, the puerperal uterus on section appears ischemic when compared with the reddish-purple hyperemic pregnant organ. After the first 2 days, the uterus begins to shrink, so that within 2 weeks it has descended into the cavity of the true pelvis. It regains its previous nonpregnant size within about 4 weeks. The immediately postpartum uterus weighs approximately 1000 g. As the consequence of involution, 1 week later it weighs about 500 g, decreasing at the end of the second week to about 300 g, and soon thereafter to 100 g or less. The total number of muscle cells does not decrease appreciably; instead, the individual cells decrease markedly in size. The involution of the connective tissue framework occurs equally rapidly.
Because separation of the placenta and membranes involves the spongy layer, the decidua basalis remains in the uterus. The decidua that remains has striking variations in thickness, an irregular jagged appearance, and is infiltrated with blood, especially at the placental site.
AFTERPAINS. In primiparas the puerperal uterus tends to remain tonically contracted. Particularly in multiparas, the uterus often contracts vigorously at intervals, giving rise to afterpains. Occasionally these pains are severe enough to require an analgesic. Afterpains are noticeable particularly when the infant suckles, likely because of oxytocin release. Usually, they decrease in intensity and become mild by the third postpartum day.
Conventional obstetrical wisdom has for many years taught that lochia lasted for approximately 2 weeks after delivery. Recent studies, however, have indicated that lochia persists for up to 4 weeks and may stop and resume up to 56 days after delivery (Oppenheimer and colleagues, 1986; Visness and co-workers, 1997). Maternal age, parity, infant weight, and breast feeding do not influence the duration of lochia.
In some centers, it is routine to prescribe an oxytocic agent to hasten uterine involution by promoting uterine contractility. This also presumably diminishes bleeding complications. Newton and Bradford (1961), however, concluded that after the period immediately following delivery, routine administration of intramuscular oxytocin to normal women was of no value in decreasing blood loss or hastening uterine involution.
Within 2 or 3 days after delivery, the remaining decidua becomes differentiated into two layers. The superficial layer becomes necrotic, and it is sloughed in the lochia. The basal layer adjacent to the myometrium remains intact and is the source of new endometrium. The endometrium arises from proliferation of the endometrial glandular remnants and the stroma of the interglandular connective tissue.
Endometrial regeneration is rapid, except at the placental site. Within a week or so, the free surface becomes covered by epithelium, and the entire endometrium is restored during the third week. Sharman (1953) identified fully restored endometrium in all biopsy specimens obtained from the 16th postpartum day onward. So-called endometritis identified histologically during the puerperium is only part of the normal reparative process. Similarly, in almost half of postpartum women, fallopian tubes, between 5 and 15 days, demonstrate microscopical inflammatory changes characteristic of acute salpingitis. This, however, is not infection, but only part of the involutional process (Andrews, 1951).
SUBINVOLUTION. This term describes an arrest or re-tardation of involution, the process by which the puerperal uterus is normally restored to its original proportions. It is accompanied by prolongation of lochial discharge and irregular or excessive uterine bleeding and sometimes by profuse hemorrhage. On bimanual examination, the uterus is larger and softer than normal for the particular period of the puerperium. Among the recognized causes of subinvolution are retention of placental fragments and pelvic infection. Because most cases of subinvolution result from local causes, they are usually amenable to early diagnosis and treatment. Ergonovine (Ergotrate) or methylergonovine (Methergine), 0.2 mg every 3 to 4 hours for 24 to 48 hours, is recommended by some clinicians, but its efficacy is questionable. On the other hand, metritis responds to oral antimicrobial therapy. Wager and colleagues (1980) reported that almost a third of cases of later postpartum uterine infection are caused by Chlamydia trachomatis; thus tetracycline therapy may be appropriate.
Andrew and colleagues (1989) described 25 cases of hemorrhage between 7 and 40 days postpartum associated with noninvoluted uteroplacental arteries. These abnormal arteries were characterized by no detectable endothelial lining and the vessels were filled with thrombi. Periauricular trophoblasts were also present in the walls of these vessels and the authors postulated that subinvolution, at least with regard to the placental vessels, may represent an aberrant interaction between uterine cells and trophoblast.
PLACENTAL SITE INVOLUTION. According to Williams (1931), complete extrusion of the placental site takes up to 6 weeks. This process is of great clinical importance, for when it is defective, late-onset puerperal hemorrhage may ensue. Immediately after delivery, the placental site is about the size of the palm of the hand, but it rapidly decreases thereafter. By the end of the second week, it is 3 to 4 cm in diameter. Within hours of delivery, the placental site normally consists of many thrombosed vessels that ultimately undergo the typical organization of a thrombus.
Williams (1931) explained involution of the placental site as follows:
Involution is not effected by absorption in situ, but rather by a process of exfoliation which is in great part brought about by the undermining of the implantation site by growth of endometrial tissue. This is affected partly by extension and downgrowth of endometrium from the margins of the placental site and partly by the development of endometrial tissue from the glands and stroma left in the depths of the decidua basalis after placental separation. Such exfoliation should be regarded as very conservative, and as a wise provision; otherwise great difficulty might be experienced in sloughing obliterated arteries and organized thrombi which, if they remained in situ, would soon convert a considerable part of the uterine mucosa and subjacent myometrium into a mass of scar tissue.
Anderson and Davis (1968) concluded that placental site exfoliation is brought about as the consequence of sloughing of infarcted and necrotic superficial tissues followed by a reparative process.
Anlagen of mammary glands are contained in ectodermal ridges that form on the ventral surface of the embryo and extend laterally from forelimb to hindlimb. The multiple pairs of buds normally disappear from the embryo except for one pair in the pectoral region that eventually develops into the two mammary glands . At times, however, the buds elsewhere may not completely disappear, but instead they may participate to an amazing degree in the pattern of growth that characterizes the two normal mammary glands.
At midpregnancy, each of the two fetal mammary buds destined to form the breasts begins to grow and divide. This results in the formation of 15 to 25 secondary buds that provide the basis for the duct system in the mature breast. Each secondary bud elongates into a cord, bifurcates, and differentiates into two concentric layers of cuboidal cells and a central lumen. The inner layer of cells eventually gives rise to the secretory epithelium, which synthesizes the milk. The outer cell layer becomes myoepithelium, which provides the mechanism for milk ejection .
Thelarche is the onset of rapid breast growth that begins about the time of puberty when estrogen production rises. The previously infantile mammary glands respond to estrogen with growth and development of mammary ducts and fat deposition. With ovulation, progesterone stimulates development of the alveoli for future lactation.
Anatomically, each mature mammary gland is composed of 15 to 25 lobes that arose from the secondary buds described previously. The lobes are arranged radially and are separated from one another by varying amounts of fat. Each lobe consists of several lobules, which in turn are made up of large numbers of alveoli Every alveolus is provided with a small duct that joins others to form a single larger duct for each lobe. These lactiferous ducts open separately upon the nipple, where they may be distinguished as minute but distinct orifices. The alveolar secretory epithelium synthesizes the various milk constituents.
LACTATION. Colostrum is the deep lemon-yellow colored liquid secreted initially by the breasts. It usually can be expressed from the nipples by the second postpartum day.
Compared with mature milk, colostrum contains more minerals and protein, much of which is globulin, but less sugar and fat. Colostrum nevertheless contains large fat globules in so-called colostrum corpuscles. These are thought by some investigators to be epithelial cells that have undergone fatty degeneration and by others to be mononuclear phagocytes containing fat. Colostrum secretion persists for about 5 days, with gradual conversion to mature milk during the ensuing 4 weeks. Antibodies are demonstrable in the colostrum, and its content of immunoglobulin A may offer protection for the newborn against enteric pathogens. Other host resistance factors, as well as immunoglobulins, are found in human colostrum and milk. These include complement, macrophages, lymphocytes, lactoferrin, lacto-peroxidase, and lysozymes.
Human milk is a suspension of fat and protein in a carbohydrate-mineral solution. A nursing mother easily makes 600 mL of milk per day. Milk is isotonic with plasma, with lactose accounting for half of the osmotic pressure. Major proteins, including a-lactalbumin, ß-lactoglobulin, and casein, are also present. Essential amino acids are derived from blood, and nonessential amino acids are derived in part from blood or synthesized in the mammary gland. Most milk proteins are unique and not found elsewhere. Whey has been shown to contain large amounts of interleukin-6 (Saito and co-workers, 1991). Peak levels of this cytokine were found in colostrum, and there was a positive correlation between its concentration and the number of mononuclear cells in human milk. Additionally, interleukin-6 was associated closely with local immunoglobulin A production by the breast. Prolactin appears to be actively secreted into breast milk (Yuen, 1988). Epidermal growth factor (EGF) has also been identified in human milk (Koldovsky and associates, 1991; McCleary, 1991). Because this factor is not destroyed by gastric proteolytic enzymes, it may be absorbed orally and promote growth and maturation of intestinal mucosa.
There are major changes in milk composition by 30 to 40 hours postpartum, including a sudden increase of lactose concentration. Lactose synthesis from glucose in alveolar secretory cells is catalyzed by lactose synthase (Fig. 17-4). Some lactose enters the maternal circulation and is excreted by the kidney. This may be misinterpreted as glucosuria unless specific glucose oxidase is used in testing. Fatty acids are synthesized in the alveoli from glucose and are secreted by an apocrine-like process.
All vitamins except vitamin K are found in human milk, but in variable amounts, and maternal dietary supplementation increases the secretion of most of these (American Academy of Pediatrics, 1981). Vitamin K administration to the infant soon after delivery is required to prevent hemorrhagic disease of the newborn.
Human milk contains a low iron concentration and maternal iron stores do not seem to influence the amount of iron in breast milk. Therefore, the use of supplemental iron-fortified infant formulas, or a weaning formula also fortified with iron, is recommended (American Academy of Pediatrics, 1997). Such formulas apparently have eliminated iron-deficiency anemia during childhood (Yip and associates, 1987). These formulas are well tolerated by most infants and there is no evidence that they impair absorption of zinc or copper (Nelson and associates, 1988; Yip and colleagues, 1985).
Mennella and Beauchamp (1991) documented what experienced nursing mothers have long known: breast-fed infants are aware of what their mothers eat and drink. They studied the effects of maternal ethanol ingestion equivalence to one can of beer. This caused the infants to suck more frequently during the first minute of feeding, but ultimately they consumed significantly less milk.
The mammary gland, like the thyroid gland, concentrates iodine and several other minerals, including gallium, technetium, indium, and possibly sodium. Radioactive isotopes of these minerals should not be given to nursing women because they rapidly appear in breast milk. The American Academy of Pediatrics (1997) recommends consultation with a nuclear medicine physician before performing a diagnostic study, so that a radionuclide with the shortest excretion time in breast milk can be used. They further recommend that the mother pump her breasts before the study and store enough milk in a freezer for feeding the infant. After the study, she should pump her breasts to maintain milk production, but discard all milk produced during the time that radioactivity is present. This ranges from 15 hours up to 2 weeks, depending upon the isotope used.
The approximate concentrations of the more important components of human colostrum, mature human milk, and cow milk are presented in Table 17-1. These concentrations vary depending upon maternal diet and when studied in the puerperium (Brasil and co-workers, 1991; Giovannini and colleagues, 1991; Ogunleye and associates, 1991). Gestational weight gain has little, if any, impact on the subsequent milk quantity or quality (Institute of Medicine, 1990).
ENDOCRINOLOGY OF LACTATION.
The precise humoral and neural mechanisms involved in lactation are complex. Progesterone, estrogen, and placental lactogen, as well as prolactin, cortisol, and insulin, appear to act in concert to stimulate the growth and development of the milk-secreting apparatus of the mammary gland (Porter, 1974). With delivery, there is an abrupt and profound decrease in the levels of progesterone and estrogen, which removes the inhibitory influence of progesterone on the production of a-lactalbumin by the rough endoplasmic reticulum . The increased a-lactalbumin serves to stimulate lactose synthase and ultimately increased milk lactose. Progesterone withdrawal also allows prolactin to act unopposed in its stimulation of a-lactalbumin production.
The intensity and duration of subsequent lactation are controlled, in large part, by the repetitive stimulus of nursing. Prolactin is essential for lactation; women with extensive pituitary necrosis, as in Sheehan syndrome, do not lactate. Although plasma prolactin falls after delivery to lower levels than during pregnancy, each act of suckling triggers a rise in levels (McNeilly and associates, 1983). Presumably a stimulus from the breast curtails the release of prolactin-inhibiting factor from the hypothalamus; this, in turn, transiently induces increased prolactin secretion.
The neurohypophysis, in pulsatile fashion, secretes oxytocin. This stimulates milk expression from a lactating breast by causing contraction of myoepithelial cells in the alveoli and small milk ducts. Milk ejection, or "letting down," is a reflex initiated especially by suckling, which stimulates the neurohypophysis to liberate oxytocin (McNeilly and associates, 1983). It may be provoked even by the cry of the infant or inhibited by fright or stress.
In women who continue lactating but who resume ovulation, there are acute alterations in breast milk composition 5 to 6 days before and 6 to 7 days following ovulation (Hartmann and Prosser, 1984). These changes are abrupt and characterized by increased concentrations of sodium and chloride, along with decreased potassium, lactose, and glucose concentrations. In women who become pregnant but who continue to breast feed, milk composition undergoes progressive alterations suggesting gradual loss of metabolic and secretory breast activity
Nipple care is also important during breast-feeding. The nipples should be washed with water and exposed to the air for 15 to 20 minutes after each feeding. A water-based cream such as lanolin or vitamin A and D ointment may be applied if the nipples are tender.
Mastitis is an uncommon complication of breast-feeding and usually develops 2 to 4 weeks after beginning breast-feeding. The first symptoms are usually slight fever and chills. These are followed by redness of a segment of the breast, which becomes indurated and painful. The etiologic agent is usually Staphylococcus aureus, which originates from the infant’s oral pharynx. Milk should be obtained from the breast for the culture and sensitivity, and mother should be started on a regimen of antibiotics immediately. Because the majority of staphylococcal organisms are penicillinase-producing, a penicillinase-resistant antibiotic, such as dicloxacillin, should be used. Breast-feeding should be discontinued, and an appropriate antibiotic should be continued for 7 to 10 days. If a breast abscess ensues, it should be surgically drained. A breast pump can be used to maintain lactation until the infection has cleared, but the milk should be discarded. The infant, along with other family members, should be evaluated for staphylococcal infections that may be source of reinfection if breast-feeding is resumed.