Labor and Birth Processes

Labor and Birth Processes. Clinical Management

Prepared by assistant professor N.Petrenko, MD, PhD

 

LEARNING OBJECTIVES

• Explain the five factors that affect the labor process.

• Describe the anatomic structure of the bony pelvis.

• Recognize the normal measurements of the diameters of the pelvic inlet, cavity, and outlet.

• Review the anatomy and the normal measurements of the fetal skull.

• Explain the significance of molding of the fetal head during labor.

• Describe the cardinal movements of the mechanism of labor.

• Assess the maternal anatomic and physiologic adaptations to labor.

• Describe fetal adaptations to labor.

 

KEY TERMS AND DEFINITIONS

asynclitism Oblique presentation of the fetal head at the superior strait of the pelvis; the pelvic planes and those of the fetal head are not parallel

attitude Relation of fetal parts to each other in the uterus (e.g., all parts flexed, or all parts flexed except neck is extended)

biparietal diameter Largest transverse diameter of the fetal head; extends from one parietal bone to the other

bloody show Vaginal discharge that originates in the cervix and consists of blood and mucus; increases as cervix dilates during labor

dilation Stretching of the external os from an opening a few millimeters in size to an opening large enough to allow the passage of the fetus

effacement Thinning and shortening or obliteration of the cervix that occurs during late pregnancy or labor or both

engagement In obstetrics, the entrance of the fetal presenting part into the superior pelvic strait and the beginning of the descent through the pelvic canal

Ferguson reflex Reflex contractions (urge to push) of the uterus after stimulation of the cervix

fontanels Broad areas, or soft spots, consisting of a strong band of connective tissue contiguous with cranial bones and located at the junctions of the bones

lie Relationship existing between the long axis of the fetus and the long axis of the mother; in a longitudinal lie, the fetus is lying lengthwise or vertically, whereas in a transverse lie, the fetus is lying crosswise or horizontally in the uterus

lightening Sensation of decreased abdominal distention produced by uterine descent into the pelvic cavity as the fetal presenting part settles into the pelvis; usually occurs 2 weeks before the onset of labor in nulliparas

molding Overlapping of cranial bones or shaping of the fetal head to accommodate and conform to the bony and soft parts of the mother's birth canal during labor

position Relationship of a reference point on the presenting part of the fetus, such as the occiput, sacrum, chin, or scapula, to its location in the front, back, or sides of the maternal pelvis

presentation That part of the fetus that first enters the pelvis and lies over the inlet; may be head, face, breech, or shoulder

presenting part That part of the fetus that lies closest to the internal os of the cervix

station Relationship of the presenting fetal part to an imaginary line drawn between the ischial spines of the pelvis

suboccipitobregmatic diameter Smallest diameter of the fetal head; follows a line drawn from the middle of the anterior fontanel to the undersurface of the occipital bone

Valsalva maneuver Any forced expiratory effort against a closed airway such as holding one's breath and tightening the abdominal muscles (e.g., pushing during the second stage of labor)

vertex Crown, or top, of the head


During late pregnancy, the woman and fetus prepare for the labor process. The fetus has grown and developed in preparation for extrauterine life. The woman has undergone various physiologic adaptations during pregnancy that prepare her for birth and motherhood. Labor and birth represent the end of pregnancy, the beginning of extrauterine life for the newborn, and a change in the lives of the family. This chapter discusses the factors affecting labor, the process involved, the normal progression of events, and the adaptations made by both the woman and fetus.


FACTORS AFFECTING LABOR

At least five factors affect the process of labor and birth. These are easily remembered as the five P's: passenger (fetus and placenta), passageway (birth canal), powers (contractions), position of the mother, and psychologic response. The first four factors are presented here as the basis of understanding the physiologic process of labor. The fifth factor is discussed in Chapter 12. Other factors such as place of birth, preparation, type of provider, and procedures implemented may be important as well (VandeVusse, 1999).

 

PASSENGER

The way the passenger, or fetus, moves through the birth canal is determined by several interacting factors: the size of the fetal head, fetal presentation, fetal lie, fetal attitude, and fetal position. Because the placenta must also pass through the birth canal, it can be considered a passenger along with the fetus. However, the placenta rarely impedes the process of labor in normal vaginal birth, except in cases of placenta previa.

 

Size of the fetal head

Fig. 1 Fetal head at term. A, Bones. B, Sutures and fontanels

 

Because of its size and relative rigidity, the fetal head has a major effect on the birth process. The fetal skull is composed of two parietal bones, two temporal bones, the frontal bone, and the occipital bone (Fig. 1, A). These bones are united by membranous sutures: the sagittal, lambdoidal, coronal, and frontal (Fig. 1, B). Membranefilled spaces called fontanels are located where the sutures intersect. During labor, after rupture of membranes, palpation of fontanels and sutures during vaginal examination reveals fetal presentation, position, and attitude.

The two most important fontanels are the anterior and posterior ones (see Fig. 1, B). The larger of these, the anterior fontanel, is diamond shaped, is approximately 3 by 2 cm in size, and lies at the junction of the sagittal, coronal, and frontal sutures. It closes by 18 months after birth. The posterior fontanel lies at the junction of the sutures of the two parietal bones and the one occipital bone, is triangular in shape, and is approximately 1 by 2 cm in size. It closes 6 to 8 weeks after birth.

Sutures and fontanels make the skull flexible to accommodate the infant brain, which continues to grow for some time after birth. Because the bones are not firmly united, however, slight overlapping of the bones, or molding of the shape of the head, occurs during labor. This capacity of the bones to slide over one another also permits adaptation to the various diameters of the maternal pelvis. Molding can be extensive, but the heads of most newborns assume their normal shape within 3 days of birth.

Although the size of the fetal shoulders may affect passage, their position can be altered relatively easily during labor; therefore one shoulder may occupy a lower level than the other. This creates a shoulder diameter that is smaller than the skull, facilitating passage through the birth canal. The circumference of the fetal hips is usually small enough not to create problems.

Fetal presentation

 

Fig. 2 Examples of fetal vertex (occiput) presentations in relation to front, back, or side of maternal pelvis.

 

Fig. 3 Fetal presentations. A-C, Breech (sacral) presentation. D, Shoulder presentation.

 

Presentation refers to the part of the fetus that enters the pelvic inlet first and leads through the birth canal during labor at term. The three main presentations are cephalic presentation (head first), occurring in 96% of births (Fig. 2); breech presentation (buttocks or feet first), occurring in 3% of births (Fig. 3, A-C); and shoulder presentation, seen in 1% of births (Fig. 3, D). Presenting part refers to that part of the fetal body first felt by the examining finger during a vaginal examination. In a cephalic presentation, the presenting part is usually the occiput; in a breech presentation, it is the sacrum; in the shoulder presentation, it is the scapula. When the presenting part is the occiput, the presentation is noted as vertex (see Fig. 2). Factors that determine the presenting part include fetal lie, fetal attitude, and extension or flexion of the fetal head.

 

Fetal lie

Lie is the relationship of the long axis (spine) of the fetus to the long axis (spine) of the mother. There are two primary lies: longitudinal, or vertical, in which the long axis of the fetus is parallel with the long axis of the mother (see Fig. 11-2); and transverse, horizontal, or oblique, in which the long axis of the fetus is at a right angle diagonal to the long axis of the mother (see Fig. 3, D). Longitudinal lies are either cephalic or breech presentations, depending on the fetal structure that first enters the mother's pelvis. Vaginal birth cannot occur when the fetus stays in a transverse lie. An oblique lie is less common and usually converts to a longitudinal or transverse lie during labor (Cunningham et al., 2001).

 

Fetal attitude

Fig. 4 Diameters of the fetal head at term. A, Cephalic presentations: occiput, vertex, and sinciput; and cephalic diameters: suboccipitobregmatic, occipitofrontal, and occipitomental. B, Biparietal diameter.

 

Fig. 5 Head entering pelvis. Biparietal diameter is indicated with shading (9.25 cm). A, Suboccipitobregmatic diameter: complete flexion of head on chest so that smallest diameter enters. B, Occipitofrontal diameter: moderate extension (military attitude) so that large diameter enters. C, Occipitomental diameter: marked extension (deflection) so that largest diameter, which is too large to permit head to enter pelvis, is presenting.

 

Attitude is the relationship of the fetal body parts to each other. The fetus assumes a characteristic posture (attitude) in utero partly because of the mode of fetal growth and partly because of the way the fetus conforms to the shape of the uterine cavity. Normally, the back of the fetus is rounded so that the chin is flexed on the chest, the thighs are flexed on the abdomen, and the legs are flexed at the knees. The arms are crossed over the thorax, and the umbilical cord lies between the arms and the legs. This attitude is termed general flexion (see Fig. 2).

Deviations from the normal attitude may cause difficulties in childbirth. For example, in a cephalic presentation, the fetal head may be extended or flexed in a manner that presents a head diameter that exceeds the limits of the maternal pelvis, leading to prolonged labor, forceps- or vacuum-assisted birth, or cesarean birth.

Certain critical diameters of the fetal head are usually measured. The biparietal diameter, which is approximately 9.25 cm at term, is the largest transverse diameter and an important indicator of fetal head size (Fig. 4, B). In a well-flexed cephalic presentation, the biparietal diameter will be the widest part of the head entering the pelvic inlet. There are several anteroposterior diameters, but the smallest and the most critical one is the suboccipitobregmatic diameter (approximately 9.5 cm at term). When the head is in complete flexion, this diameter allows the fetal head to pass through the true pelvis easily (Figs. 4, A, and 5, A). As the head is more extended, the anteroposterior diameter widens and the head may not be able to enter the true pelvis (Fig. 5, B and C).

 

Fetal position

The presentation or presenting part indicates that portion of the fetus that overlies the pelvic inlet. Position is the relationship of the presenting part (occiput, sacrum, mentum [chin], or sinciput [deflexed vertex]) to the four quadrants of the mother's pelvis (see Fig. 2). Position is denoted by a three-letter abbreviation. The first letter of the abbreviation denotes the location of the presenting part in the right (R) or left (L) side of the mother's pelvis. The middle letter stands for the specific presenting part of the fetus (0 for occiput, S for sacrum, M for mentum [chin], and Sc for scapula [shoulder]). The third letter stands for the location of the presenting part in relation to the anterior (A), posterior (P), or transverse (T) portion of the maternal pelvis. For example, ROA means that the occiput is the presenting part and is located in the right anterior quadrant of the maternal pelvis (see Fig. 2). LSP means that the sacrum is the presenting part and is located in the left posterior quadrant of the maternal pelvis (see Fig. 3).

 

 

Fig. 6 Stations of presenting part, or degree of descent. Biparietal diameter of the fetal head is just below the level of the ischial spines, between station 0 and station +1.

 

Station is the relationship of the presenting part of the fetus to an imaginary line drawn between the maternal ischial spines and is a measure of the degree of descent of the presenting part of the fetus through the birth canal. The placement of the presenting part is measured in centimeters above or below the ischial spines (Fig. 6). For example, when the lowermost portion of the presenting part is 1 cm above the spines, it is noted as being minus (—) 1. At the level of the spines, the station is referred to as 0 (zero). When the presenting part is 1 cm below the spines, the station is said to be plus (+) 1. Birth is imminent when the presenting part is at +4 to +5 cm. The station of the presenting part should be determined when labor begins so that the rate of descent of the fetus during labor can be accurately determined.

Engagement is the term used to indicate that the largest transverse diameter of the presenting part (usually the biparietal diameter) has passed through the maternal pelvic brim or inlet into the true pelvis and usually corresponds to station 0. Engagement often occurs in the weeks just before labor begins in primigravidas and may occur before labor or during labor in multigravidas. Engagement can be determined by abdominal or vaginal examination.

 

PASSAGEWAY

The passageway, or birth canal, is composed of the mother's rigid bony pelvis and the soft tissues of the cervix, pelvic floor, vagina, and introitus (the external opening to the vagina). Although the soft tissues, particularly the muscular layers of the pelvic floor, contribute to vaginal birth of the fetus, the maternal pelvis plays a far greater role in the labor process because the fetus must successfully accommodate itself to this relatively rigid passageway. Therefore the size and shape of the pelvis must be determined before childbirth begins.

Bony pelvis

 

Fig . 7 Female pelvis. A, Pelvic brim above. B, Pelvic outlet from below.

 

The anatomy of the bony pelvis is described in Chapter 4. The following discussion focuses on the importance of pelvic configurations as they relate to the labor process. (It may be helpful to refer to Fig. 4.)

The bony pelvis is formed by the fusion of the ilium, ischium, pubis, and sacral bones. The four pelvic joints are the symphysis pubis, the right and left sacroiliac joints, and the sacrococcygeal joint (Fig. 7, A). The bony pelvis is separated by the brim, or inlet, into two parts: the false pelvis and the true pelvis. The false pelvis is that part above the brim and plays no part in childbearing. The true pelvis, that part involved in birth, is divided into three planes: the inlet, or brim; the midpelvis, or cavity; and the outlet.

The pelvic inlet, which is the upper border of the true pelvis, is formed anteriorly by the upper margins of the pubic bone, laterally by the iliopectineal lines along the innominate bones, and posteriorly by the anterior, upper margin of the sacrum and the sacral promontory.

The pelvic cavity, or midpelvis, is a curved passage having a short anterior wall and a much longer concave posterior wall. It is bounded by the posterior aspect of the symphysis pubis, the ischium, a portion of the ilium, the sacrum, and the coccyx.

The pelvic outlet is the lower border of the true pelvis. Viewed from below, it is ovoid, somewhat diamond shaped, bounded by the pubic arch anteriorly, the ischial tuberosities laterally, and the tip of the coccyx posteriorly (Fig. 7, B). In the latter part of pregnancy the coccyx is movable (unless it has been broken in a fall during skiing or skating, for example, and has fused to the sacrum during healing).

 

Fig. 8 Pelvic cavity. A, Inlet and midplane. Outlet not shown. B, Cavity of true pelvis. C, Note curve of sacrum and axis of birth canal.

 

The pelvic canal varies in size and shape at various levels. The diameters at the plane of the pelvic inlet, midpelvis, and outlet, plus the axis of the birth canal (Fig. 8), determine whether vaginal birth is possible and the manner by which the fetus may pass down the birth canal.

The subpubic angle, which determines the type of pubic arch, together with the length of the pubic rami and the intertuberous diameter, is of great importance. Because the fetus must first pass beneath the pubic arch, a narrow subpubic angle will be less accommodating than a rounded, wide arch. The method of measurement of the subpubic arch is shown in Fig. 9. A summary of obstetric measurements is given in Table 1.

 

 

Fig. 9 Estimation of angle of subpubic arch. Using both thumbs, examiner externally traces descending ramidown to tuberosities. (From Barkauskas, V., Baumann, L, & Darling-Fisher, C. [2002], Health and physical assessment [3rd ed.]. St. Louis: Mosby.)

 

Table 1 Obstetric Measurements

PLANE

DIAMETER

MEASUREMENTS

Inlet (superior strait)

Conjugates

Diagonal

Obstetric: measurement that determines whether presenting part can engage or enter superior strait

True (vera) (anteroposterior)

 

 

12.5-13 cm

1.5-2 cm less than diagonal

(radiographic)

 

 

 

11 cm (12.5) (radiographic)

 

 

Length of diagonal conjugate (solid colored line), obstetric conjugate (broken colored line), and true conjugate (black line)*

Midplane

Transverse diameter (interspinous diameter)

The midplane of the pelvis normally is its largest plane and the one of greatest diameter.

10.5 cm

Measurement of interspinous diameter*

Outlet

Transverse diameter (intertuberous diameter) (biischial)

The outlet presents the smallest plane of the pelvic canal.

>8 cm

Use of Thorn's pelvimeter to measure

intertuberous diameter*

 

 

The four basic types of pelvis are classified as follows:

1. Gynecoid (the classic female type)

2. Android (resembling the male pelvis)

3. Anthropoid (resembling the pelvis of anthropoid apes)

4. Platypelloid (the flat pelvis)

The gynecoid pelvis is the most common, with major gynecoid pelvic features present in 50% of all women. Anthropoid and android features are less common, and platypelloid pelvic features are the least common. Mixed types of pelves are more common than pure types (Cunningham et al., 2001). Examples of pelvic variations and their effects on mode of birth are given in Table 2.

 

TABLE 2 Comparison of Pelvic Types

 

Assessment of the bony pelvis can be performed during the first prenatal evaluation and need not be repeated if the pelvis is of adequate size and suitable shape. In the third trimester of pregnancy, the examination of the bony pelvis may be more thorough and the results more accurate because there is relaxation and increased mobility of the pelvic joints and ligaments due to hormonal influences. Widening of the joint of the symphysis pubis and the resulting instability may cause pain in any or all of the pelvic joints.

Because the examiner does not have direct access to the bony structures and because the bones are covered with varying amounts of soft tissue, estimates of size and shape are approximate. Precise bony pelvis measurements can be determined by use of computed tomography, ultrasound, or x-ray films. However, x-ray examination is rarely done during pregnancy because the x-rays may damage the developing fetus.

 

Soft tissues

The soft tissues of the passageway include the distensible lower uterine segment, cervix, pelvic floor muscles, vagina, and introitus. Before labor begins, the uterus is composed of the uterine body (corpus) and cervix (neck). After labor has begun, uterine contractions cause the uterine body to have a thick and muscular upper segment and a thin-walled, passive, muscular lower segment. A physiologic retraction ring separates the two segments (Fig. 10). The lower uterine segment gradually distends to accommodate the intrauterine contents as the wall of the upper segment thickens and its accommodating capacity is reduced. The contractions of the uterine body thus exert downward pressure on the fetus, pushing it against the cervix.

The cervix effaces (thins) and dilates (opens) sufficiently to allow the first fetal portion to descend into the vagina. As the fetus descends, the cervix is actually drawn upward and over this first portion.

 

 

Fig. 10 Uterus in normal labor in early first stage (A) and in second stage (B). Passive segment is derived from lower uterine segment (isthmus) and cervix, and physiologic retraction is derived from anatomic internal os. C, Uterus in abnormal labor in second-stage dystocia. Pathologic retraction (Bandl's) ring that forms under abnormal conditions develops from the physiologic ring.

 

The pelvic floor is a muscular layer that separates the pelvic cavity above from the perineal space below. This structure helps the fetus rotate anteriorly as it passes through the birth canal. As noted earlier, the soft tissues of the vagina develop throughout pregnancy until at term the vagina can dilate to accommodate the fetus and permit passage of the fetus to the external world.

 

POWERS

Involuntary and voluntary powers combine to expel the fetus and the placenta from the uterus. Involuntary uterine contractions, termed the primary powers, signal the beginning of labor. Once the cervix has dilated, voluntary bearingdown efforts by the woman, termed the secondary powers, augment the force of the involuntary contractions.

 

Primary powers

The involuntary contractions originate at certain pacemaker points in the thickened muscle layers of the upper uterine segment. From the pacemaker points, contractions move downward over the uterus in waves, separated by short rest periods. Terms used to describe these involuntary contractions include frequency (the time from the beginning of one contraction to the beginning of the next), duration (length of contraction), and intensity (strength of contraction).

The primary powers are responsible for the effacement and dilation of the cervix and descent of the fetus. Effacement of the cervix means the shortening and thinning of the cervix during the first stage of labor. The cervix, normally 2 to 3 cm long and approximately 1 cm thick, is obliterated or "taken up" by a shortening of the uterine muscle bundles during the thinning of the lower uterine segment that occurs in advancing labor. Only a thin edge of the cervix can be palpated when effacement is complete. Effacement generally is advanced in first-time term pregnancy before more than slight dilation occurs. In subsequent pregnancies, effacement and dilation of the cervix tend to progress together. Degree of effacement is expressed in percentages from 0% to 100% (e.g., a cervix is 50% effaced) (Fig. 11, A-Q.

 

Fig. 11 Cervical effacement and dilation. Note how cervix is drawn up around presenting part (internal os). Membranes are intact and head is not well applied to cervix. A, Before labor. B, Early effacement. C, Complete effacement (100%). Head is well applied to cervix. D, Complete dilation (10 cm). There is some overlapping of cranial bones and membranes are still intact.

 

Dilation of the cervix is the enlargement or widening of the cervical opening and the cervical canal that occurs once labor has begun. The diameter of the cervix increases from less than 1 cm to full dilation (approximately 10 cm) to allow birth of a term fetus. When the cervix is fully dilated (and completely retracted), it can no longer be palpated (Fig. 11, D). Full cervical dilation marks the end of the first stage of labor.

Dilation of the cervix occurs by the drawing upward of the musculofibrous components of the cervix that is caused by strong uterine contractions. Pressure exerted by the amniotic fluid while the membranes are intact or by the force applied by the presenting part also can promote cervical dilation. Scarring of the cervix as a result of prior infection or surgery may slow cervical dilation.

In the first and second stages of labor, increased intrauterine pressure caused by contractions exerts pressure on the descending fetus and the cervix. When the presenting part of the fetus reaches the perineal floor, mechanical stretching of the cervix occurs. Stretch receptors in the posterior vagina cause release of endogenous oxytocin that triggers the maternal urge to bear down, or the Ferguson reflex.

Uterine contractions are usually independent from external forces. For example, laboring women who are paraplegic will have normal but painless uterine contractions (Cunningham et al., 2001). Uterine contractions may decrease temporarily in frequency and intensity if narcotic analgesic medication or epidural analgesia is given early in labor (Alexander et al., 1998). The exact relationship between prolonged labor and epidural analgesia continues to be investigated (Thompson et al., 1998).

 

Secondary powers

As soon as the presenting part reaches the pelvic floor the contractions change in character and become expulsive. The laboring woman experiences an involuntary urge to push. She uses secondary powers (bearing-down efforts) to aid in expulsion of the fetus as she contracts her diaphragm and abdominal muscles and pushes. These bearing-down efforts result in increased intraabdominal pressure that compresses the uterus on all sides and adds to the power of the expulsive forces.

The secondary powers have no effect on cervical dilation, but they are of considerable importance in the expulsion of the infant from the uterus and vagina after the cervix is fully dilated. Studies have shown that pushing in the second stage is more effective and the woman is less fatigued when she begins to push only after she has the urge to do so rather than beginning to push when she is fully dilated without an urge to do so (Roberts & Woolley, 1996).

When and how a woman pushes in second stage is a much-debated topic. Studies have investigated the effects of spontaneous bearing-down efforts, directed pushing, delayed pushing, Valsalva (closed glottis and prolonged bearing down) pushing, and open glottis pushing (Peterson & Besuner, 1997; Roberts & Woolley, 1996; Sampselle, 1999; Thomson, 1995). Although no significant differences have been found in the duration of second-stage labor, adverse consequences have been reported. Fetal hypoxia and subsequent acidosis have been associated with prolonged breath holding and forceful pushing efforts (Mayberry et al., 1999). Perineal tears have been associated with directed pushing. Continued study is needed to determine the effectiveness and appropriateness of teaching strategies used by nurses to teach pushing techniques, the suitability and effectiveness of various pushing techniques related to nonreassuring fetal heart patterns, and the standards for length of pushing in terms of maternal and fetal outcomes (Minato, 2000/2001).

 

POSITION OF THE LABORING WOMAN

Fig. 12 Positions for labor and birth. A, Positions for labor. B, Positions for giving birth.

 

Position affects the woman's anatomic and physiologic adaptations to labor. Frequent changes in position relieve fatigue, increase comfort, and improve circulation. Therefore a laboring woman should be encouraged to find positions that are most comfortable to her (Fig. 12, A) (see Research box).

 

RESEARCH Position During Second Stage Labor

Western women usually give birth in the supine or lithotomy position, which is convenient for the clinician monitoring labor. In societies where squatting is customary for rest and defecation, squatting for second-stage labor is far more common. Researchers have claimed several advantages of upright position during delivery: gravity assistance, stronger and more effective uterine contractions, lessened aorta compression and less fetal acidosis, improved fetal alignment for birth, and increased pelvic outlet diameter. Upright positions include sitting (using obstetric chair or stool), semirecumbent, kneeling, squatting (unaided or using squatting bars), or squatting (using birth cushion).

A meta-analysis of 18 randomized and quasi-randomized trials compared labor outcomes of various positions used during second-stage labor. When compared with supine positions, upright position reduced second-stage labor by a mean of 5.4 minutes. This was greatly influenced by the rather large reduction (16.9 minutes) in the two trials of the birth cushion. The birth cushion trials also influenced the reduction in assisted deliveries with upright position, compared with supine positions. Women also had a decreased episiotomy rate when they delivered upright. Researchers speculated that the increased blood loss noted in upright labor may have been influenced by the fact that the birth chair has a receptacle for lost blood, so blood loss is measured. The researchers conclude that bearing down in the second stage of labor is more efficient in the upright position.They caution that poor overall study qualities preclude definite conclusions.

IMPLICATIONS FOR PRACTICE

Upright positions during second-stage labor may have advantages over supine positions, including prevention of cesarean birth because of failure to progress. Labor nurses should be familiar with different positions for second-stage labor so that they can help women find a position that feels right to them and also causes labor to progress. Although upright positions can be maintained without special equipment, availability of birthing beds with squatting bars, birthing stools/chairs, and birthing balls would encourage birthing in a different position.

Source: Gupta, J., & Nikodem, V. (2001). Woman's position during second stage of labor (Cochrane Review). The Cochrane Library, Issue 2. Oxford: Update Software.

 

An upright position (walking, sitting, kneeling, or squatting) offers a number of advantages. Gravity can promote the descent of the fetus. Uterine contractions are generally stronger and more efficient in effacing and dilating the cervix, resulting in shorter labor (Shermer & Raines, 1997; Simkin & Ancheta, 2000).

An upright position is also beneficial to the mother's cardiac output, which normally increases during labor as uterine contractions return blood to the vascular bed. The increased cardiac output improves blood flow to the uteroplacental unit and the maternal kidneys. Cardiac output is compromised if the descending aorta and ascending vena cava are compressed during labor. Compression of these major vessels may result in supine hypotension that decreases placental perfusion. With the woman in an upright position, pressure on the maternal vessels is reduced and compression is prevented. If the woman wishes to lie down, a lateral position is suggested (Cunningham et al., 2001).

The "all fours" position (hands and knees) may be used to relieve backache if the fetus is in an occipitoposterior position and may assist in anterior rotation of the fetus and in cases of shoulder dystocia (Simkin 1995; Simkin & Ancheta, 2000).

Positioning for second-stage labor (Fig. 12, B) may be determined by the woman's preference, but it is constrained by the condition of the woman or fetus, the environment, and the health care provider's confidence in assisting in a birth in a specific position (Simkin & Ancheta, 2000). The predominant position in the United States in physician-attended births is the lithotomy position. Alternative positions and position changes are more commonly practiced by nurse-midwives (Hanson, 1998).

A woman who pushes in a semirecumbent position needs adequate body support to push effectively because her weight will be on her sacrum, moving the coccyx forward and causing a reduction in the pelvic outlet. In a sitting or squatting position, abdominal muscles work in greater synchronicity with uterine contractions during bearing-down efforts. Kneeling or squatting moves the uterus forward and aligns the fetus with the pelvic inlet and can facilitate the second stage of labor by increasing the pelvic outlet (Simkin & Ancheta, 2000).

The lateral position can be used by the woman to help rotate a fetus that is in a posterior position. It can also be used when there is a need for less force to be used during bearing down such as when there is a need to control the speed of a precipitous birth (Roberts & Woolley, 1996; Simkin & Ancheta, 2000).

There is no evidence that any of these positions suggested for second-stage labor increase the need for use of operative techniques (e.g., forceps- or vacuum-assisted birth, cesarean birth, episiotomy) or cause perineal trauma. There is also no evidence that use of any of these positions adversely affects the newborn (Mayberry et al., 2000).


PROCESS OF LABOR

Labor is the process of moving the fetus, placenta, and membranes out of the uterus and through the birth canal. Various changes take place in the woman's reproductive system in the days and weeks before labor begins. Labor itself can be discussed in terms of the mechanisms involved in the process and the stages the woman moves through.

 

SIGNS PRECEDING LABOR

In first-time pregnancies the uterus sinks downward and forward approximately 2 weeks before term, when the fetus's presenting part (usually the fetal head) descends into the true pelvis. This settling is called lightening, or "dropping," and usually happens gradually. After lightening, women feel less congested and breathe more easily. However, there is usually more bladder pressure as a result of this shift and consequently a return of urinary frequency. In a multiparous pregnancy, lightening may not take place until after uterine contractions are established and true labor is in progress.

The woman may complain of persistent low backache and sacroiliac distress as a result of relaxation of the pelvic joints. She may identify strong, frequent, but irregular uterine (Braxton Hicks) contractions.

The vaginal mucus becomes more profuse in response to the extreme congestion of the vaginal mucous membranes. Brownish or blood-tinged cervical mucus may be passed (bloody show). The cervix becomes soft (ripens) and partially effaced and may begin to dilate. The membranes may rupture spontaneously.

Other phenomena are common in the days preceding labor: (1) loss of 0.5 to 1.5 kg in weight, caused by water loss resulting from electrolyte shifts that in turn are produced by changes in estrogen and progesterone levels; and (2) a surge of energy. Women speak of having a burst of energy that they often use to clean the house and put everything in order. Less commonly, some women experience diarrhea, nausea, vomiting, and indigestion (Varney, 1997). Box 1 lists signs that may precede labor.

 

Box 1 Signs Preceding Labor

Lightening

Return of urinary frequency

Backache

Stronger Braxton Hicks contractions

Weight loss 0.5-1.5 kg

Surge of energy

Increased vaginal discharge; bloody show

Cervical ripening

Membranes may rupture

 

ONSET OF LABOR

The onset of true labor cannot be ascribed to a single cause. Many factors, including changes in the maternal uterus, cervix, and pituitary gland, are involved. Hormones produced by the normal fetal hypothalamus, pituitary, and adrenal cortex probably contribute to the onset of labor. Progressive uterine distention, increasing intrauterine pressure, and aging of the placenta seem to be associated with increasing myometrial irritability. This is a result of increased concentrations of estrogen and prostaglandins, as well as decreasing progesterone levels. The mutually coordinated effects of these factors result in the occurrence of strong, regular, rhythmic uterine contractions. Normally, the outcome of these factors working together is the birth of the fetus and the expulsion of the placenta. However, it is still not completely understood how certain alterations trigger others and how proper checks and balances are maintained.

Fetal fibronectin is a protein found in plasma and cervicovaginal secretions of pregnant women before the onset of labor. Assessment for the presence of fetal fibronectin is being used to predict the likelihood of preterm labor in women who are at increased risk for this complication. The value of detection of fetal fibronectin in management of women with preterm labor has yet to be determined (Coleman et al., 1998; Goldenberg et al, 2000).

 

STAGES OF LABOR

Labor is considered "normal" when the woman is at or near term, no complications exist, a single fetus presents by vertex, and labor is completed within 24 hours. The course of normal labor, which is remarkably constant, consists of (1) regular progression of uterine contractions, (2) effacement and progressive dilation of the cervix, and (3) progress in descent of the presenting part. Four stages of labor are recognized. These stages are discussed in greater detail, along with nursing care for the laboring woman and family.

The first stage of labor is considered to last from the onset of regular uterine contractions to full dilation of the cervix. The onset of labor is often difficult to establish because the woman may be admitted to the labor unit just before birth and the beginning of labor may be only an estimate. The first stage is much longer than the second and third combined. Great variability is the rule, however, depending on the factors discussed previously in this chapter. Full dilation may occur in less than 1 hour in some multiparous pregnancies. In first-time pregnancy, complete dilation of the cervix can take up to 20 hours. There are no absolute values for the normal length of the first stage of labor (American College of Obstetricians and Gynecologists [ACOG], 1995). Variations may reflect differences in the patient population or in clinical practice.

The first stage of labor has been divided into three phases: a latent phase, an active phase, and a transition phase. During the latent phase there is more progress in effacement of the cervix and little increase in descent. During the active phase and the transition phase there is more rapid dilation of the cervix and increased rate of descent of the presenting part.

The second stage of labor lasts from the time the cervix is fully dilated to the birth of the fetus. The second stage takes an average of 20 minutes for a multiparous woman and 50 minutes for a nulliparous woman. Labor of up to 2 hours has been considered within the normal range for the second stage, but there can be significant variations. For example, a woman who has received epidural analgesia may take up to 3 hours (Johnson & Rosenfeld, 1995). As long as there is progress and the fetal status is reassuring, the length of the second stage is usually not related to adverse perinatal outcomes (ACOG, 1995).

Simkin and Ancheta (2000) describe the latent and active phases of second-stage labor. The latent phase is a period that begins around the time of complete dilation of the uterus when the contractions are weak or not noticeable and the woman is not feeling the urge to push, is resting, or is exerting only small bearing-down efforts with contractions. The active phase is a period when contractions resume and the woman is making strong bearingdown efforts and the fetal station is advancing.

The third stage of labor lasts from the birth of the fetus until the placenta is delivered. The placenta normally separates with the third or fourth strong uterine contraction after the infant has been born. After it has separated, the placenta can be delivered with the next uterine contraction. The duration of the third stage may be as short as 3 to 5 minutes, although up to 1 hour is considered within normal limits. The risk of hemorrhage increases as the length of the third stage increases (Cunningham et al, 2001).

The fourth stage of labor arbitrarily lasts approximately 2 hours after delivery of the placenta. It is the period of immediate recovery, when homeostasis is reestablished. It serves as an important period of observation for complications, such as abnormal bleeding.

 

MECHANISM OF LABOR

Fig. 13 Cardinal movements of the mechanism of labor. Left occipitoanterior (LOA) presentation. A, Engagement and descent. B, Flexion. C, Internal rotation to occipitoanterior position (OA). D, Extension. E, External rotation beginning (restitution). F, External rotation.

 

As already discussed, the female pelvis has varied contours and diameters at different levels, and the presenting part of the passenger is large in proportion to the passage. Therefore, for vaginal birth to occur, the fetus must adapt to the birth canal during the descent. The turns and other adjustments necessary in the human birth process are termed the mechanism of labor (Fig. 13). The seven cardinal movements of the mechanism of labor that occur in a vertex presentation are engagement, descent, flexion, internal rotation, extension, external rotation (restitution), and finally birth by expulsion. Although these movements are discussed separately, in actuality a combination of movements occurs simultaneously. For example, engagement involves both descent and flexion.

 

Engagement

When the biparietal diameter of the head passes the pelvic inlet, the head is said to be engaged in the pelvic inlet (see Fig. 13, A). In most nulliparous pregnancies this occurs before the onset of active labor because the firmer abdominal muscles direct the presenting part into the pelvis. In multiparous pregnancies, in which the abdominal musculature is more relaxed, the head often remains freely movable above the pelvic brim until labor is established.

Asynclitism. The head usually engages in the pelvis in a synclitic position, one that is parallel to the anteroposterior plane of the pelvis. Frequently asynclitism occurs (the head is deflected anteriorly or posteriorly in the pelvis), which can facilitate descent because the head is being positioned to accommodate to the pelvic cavity (Fig. 14). However, extreme asynclitism can cause cephalopelvic disproportion, even in a normal-size pelvis, because the head is positioned so that it cannot descend.

 

 

Fig. 14 Synclitism and asynclitism. A, Anterior asynclitism. B, Normal synclitism. C, Posterior asynclitism.

 

Descent

Descent refers to the progress of the presenting part through the pelvis. Descent depends on at least four forces: (1) pressure exerted by the amniotic fluid, (2) direct pressure exerted by the contracting fundus on the fetus, (3) force of the contraction of the maternal diaphragm and abdominal muscles in the second stage of labor, and (4) extension and straightening of the fetal body. The effects of these forces are modified by the size and shape of the maternal pelvic planes and the size of the fetal head and its capacity to mold.

The degree of descent is measured by the station of the presenting part (see Fig. 6). As mentioned, little descent occurs during the latent phase of the first stage of labor. Descent accelerates in the active phase when the cervix has dilated to 5 to 7 cm. It is especially apparent when the membranes have ruptured.

In a first-time pregnancy descent is usually slow but steady; in subsequent pregnancies descent may be rapid. Progress in descent of the presenting part is determined by abdominal palpation (Leopold's maneuvers) and vaginal examination until the presenting part can be seen at the introitus.

 

Flexion

As soon as the descending head meets resistance from the cervix, pelvic wall, or pelvic floor, it normally flexes so that the chin is brought into closer contact with the fetal chest (see Fig. 13, B). Flexion permits the smaller suboccipitobregmatic diameter (9.5 cm) rather than the larger diameters to present to the outlet.

 

Internal rotation

The maternal pelvic inlet is widest in the transverse diameter. Therefore the fetal head passes the inlet into the true pelvis in the occipitotransverse position. The outlet is widest in the anteroposterior diameter, however. Therefore, for the fetus to exit, the head must rotate. Internal rotation begins at the level of the ischial spines but is not completed until the presenting part reaches the lower pelvis. As the occiput rotates anteriorly, the face rotates posteriorly. With each contraction the fetal head is guided by the bony pelvis and the muscles of the pelvic floor. Eventually, the occiput will be in the midline beneath the pubic arch. The head is almost always rotated by the time it reaches the pelvic floor (see Fig. 13, Q. Both the levator ani muscles and the bony pelvis are important for achieving anterior rotation. A previous childbirth injury or regional anesthesia may compromise the function of the levator sling.

 

Extension

When the fetal head reaches the perineum for birth, it is deflected anteriorly by the perineum. The occiput passes under the lower border of the symphysis pubis first, then the head emerges by extension: first the occiput, then the face, and finally the chin (see Fig. 13, D).

 

Restitution and external rotation

After the head is born, it rotates briefly to the position it occupied when it was engaged in the inlet. This movement is termed restitution (see Fig. 13, E). The 45-degree turn realigns the infant's head with her or his back and shoulders. The head can then be seen to rotate further. This external rotation occurs as the shoulders engage and descend in maneuvers similar to those of the head (see Fig. 13, F). As noted earlier, the anterior shoulder descends first. When it reaches the outlet, it rotates to the midline and is delivered from under the pubic arch. The posterior shoulder is guided over the perineum until it is free of the vaginal introitus.

 

Expulsion

After birth of the shoulders, the head and shoulders are lifted up toward the mother's pubic bone and the trunk of the baby is born by flexing it laterally in the direction of the symphysis pubis. When the baby has completely emerged, birth is complete, and the second stage of labor ends.


PHYSIOLOGIC ADAPTATIONS TO LABOR

In addition to the maternal and fetal anatomic adaptations that occur during birth, physiologic adaptations must also occur.

FETAL ADAPTATION

Several important physiologic adaptations occur in the fetus. These changes occur in fetal heart rate, fetal circulation, respiratory movements, and other behaviors.

 

Fetal heart rate

Fetal heart rate (FHR) monitoring provides reliable and predictive information about the condition of the fetus related to oxygenation. The average FHR at term is 140 beats per minute (beats/min). The normal range is 110 to 160 beats/min. Earlier in gestation the FHR is higher, with an average of approximately 160 beats/min at 20 weeks of gestation. The rate decreases progressively as the maturing fetus reaches term. However, temporary accelerations and slight early decelerations of the FHR can be expected in response to spontaneous fetal movement, vaginal examination, fundal pressure, uterine contractions, abdominal palpation, and fetal head compression. Stresses to the uterofetoplacental unit result in characteristic FHR patterns.

 

Fetal circulation

Fetal circulation can be affected by many factors, including maternal position, uterine contractions, blood pressure, and umbilical cord blood flow. Uterine contractions during labor tend to decrease circulation through the spiral arterioles and subsequent perfusion through the intervillous space. Most healthy fetuses are well able to compensate for this stress and exposure to increased pressure while moving passively through the birth canal during labor. Usually umbilical cord blood flow is undisturbed by uterine contractions or fetal position (Lowe & Reiss, 1996).

 

Fetal respiration

Certain changes stimulate chemoreceptors in the aorta and carotid bodies to prepare the fetus for initiating respirations immediately after birth (Lowe & Reiss, 1996). These changes include the following:

• Fetal lung fluid is cleared from the air passages during labor and (vaginal) birth.

• Fetal oxygen pressure (Po2) falls.

• Arterial carbon dioxide pressure (Pco2) rises.

• Arterial pH falls.

• Bicarbonate level falls.

• Fetal respiratory movements decrease during labor.

 

MATERNAL ADAPTATION

As the woman progresses through the stages of labor, various body systems adapt that cause the woman to exhibit both objective and subjective symptoms.

 

Cardiovascular changes

During each contraction, 400 ml of blood is emptied from the uterus into the maternal vascular system. This increases cardiac output by approximately 10% to 15% in the first stage and by approximately 30% to 50% in the second stage. The heart rate increases slightly.

Changes in the woman's blood pressure also occur. Blood flow, which is reduced in the uterine artery by contractions, is redirected to peripheral vessels. As a result, peripheral resistance increases, and blood pressure rises (Chamberlain & Pipkin, 1998). During the first stage of labor, uterine contractions cause systolic readings to rise by approximately 10 mm Hg. Therefore assessing blood pressure between contractions provides more accurate readings (Varney, 1997). During the second stage, contractions may cause systolic pressures to increase by 30 mm Hg and diastolic readings to increase by 25 mm Hg, with both systolic and diastolic pressures remaining somewhat elevated even between contractions. Therefore the woman already at risk for hypertension is at increased risk for complications such as cerebral hemorrhage.

Supine hypotension (see Fig. 4) occurs when the ascending vena cava and descending aorta are compressed. The laboring woman is at greater risk for supine hypotension if the uterus is particularly large because of multifetal pregnancy, hydramnios, or obesity or if the woman is dehydrated or hypovolemic. In addition, anxiety and pain, as well as some medications, can cause hypotension.

The woman should be discouraged from using the Valsalva maneuver (holding one's breath and tightening abdominal muscles) for pushing during the second stage. This activity increases intrathoracic pressure, reduces venous return, and increases venous pressure. The cardiac output and blood pressure increase and the pulse slows temporarily. During the Valsalva maneuver, fetal hypoxia may occur. The process is reversed when the woman takes a breath.

The white blood cell (WBC) count can increase (Pagana & Pagana, 2001). Although the mechanism leading to this increase in WBCs is unknown, it may be secondary to physical or emotional stress or to tissue trauma. Labor is strenuous, and physical exercise alone can increase the WBC count.

Some peripheral vascular changes occur, perhaps in response to cervical dilation or to compression of maternal vessels by the fetus passing through the birth canal. Flushed cheeks, hot or cold feet, and eversion of hemorrhoids may result.

 

Respiratory changes

Increased physical activity with greater oxygen consumption is reflected in an increase in the respiratory rate. Hyperventilation may cause respiratory alkalosis (an increase in pH), hypoxia, and hypocapnia (decrease in carbon dioxide). In the unmedicated woman in the second stage, oxygen consumption almost doubles. Anxiety also increases oxygen consumption.

 

Renal changes

During labor, spontaneous voiding may be difficult for various reasons: tissue edema caused by pressure from the presenting part, discomfort, analgesia, and embarrassment. Proteinuria of +1 is a normal finding because it can occur in response to the breakdown of muscle tissue from the physical work of labor.

 

Integumentary changes

The integumentary system changes are evident, especially in the great distensibility (stretching) in the area of the vaginal introitus. The degree of distensibility varies with the individual. Despite this ability to stretch, even in the absence of episiotomy or lacerations, minute tears in the skin around the vaginal introitus do occur.

 

Musculoskeletal changes

The musculoskeletal system is stressed during labor. Diaphoresis, fatigue, proteinuria (+1), and possibly an increased temperature accompany the marked increase in muscle activity. Backache and joint ache (unrelated to fetal position) occur as a result of increased joint laxity at term. The labor process itself and the woman's pointing her toes can cause leg cramps.

 

Neurologic changes

Sensorial changes occur as the woman moves through phases of the first stage of labor and as she moves from one stage to the next. Initially she may be euphoric. Euphoria gives way to increased seriousness, then to amnesia between contractions during the second stage, and finally to elation or fatigue after giving birth. Endogenous endorphins (a morphine-like chemical produced naturally by the body) raise the pain threshold and produce sedation. In addition, physiologic anesthesia of perineal tissues, caused by pressure of the presenting part, decreases perception of pain.

 

Gastrointestinal changes

During labor, gastrointestinal motility and absorption of solid foods are decreased, and stomach emptying time is slowed. Nausea and vomiting of undigested food eaten after onset of labor are common. Nausea and belching also occur as a reflex response to full cervical dilation. The woman may state that diarrhea accompanied the onset of labor, or the nurse may palpate the presence of hard or impacted stool in the rectum.

 

Endocrine changes

The onset of labor may be triggered by decreasing levels of progesterone and increasing levels of estrogen, prostaglandins, and oxytocin. Metabolism increases, and blood glucose levels may decrease with the work of labor. Accurate assessment of the mother and fetus during labor and birth depends on knowledge of these expected adaptations so that appropriate interventions can be implemented.


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).

SPONTANEOUS DELIVERY

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.