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. 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.
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 . 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 .
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.
FETAL BRAIN. There is a steady gestational age-related change in the appearance and function of the fetal brain. 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. 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.
In shoulder presentations, the acromion (scapula) is the portion of the fetus arbitrarily chosen for orientation with the maternal pelvis. The acromion or back of the fetus may be directed either posteriorly or anteriorly and superiorly or inferiorly. 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