Size of the Fetal Head

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. 13-1, A). These bones are united by membranous sutures: the sagittal, lambdoidal, coronal, and frontal (see Fig. 13-1, B). Membrane-filled 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 (see Fig. 13-1, B). The larger of these, the anterior fontanel, is diamond shaped, about 3 cm by 2 cm, 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 occipital bone, is triangular, and is about 1 cm by 2 cm. 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. However, because the bones are not firmly united, slight overlapping, 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 after birth.

Although the size of the fetal shoulders may affect passage, their position can be altered relatively easily during labor so 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 to not create problems.

Fetal 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. 13-2); breech presentation (buttocks, feet, or both first), occurring in 3% of births (Fig. 13-3, A-C); and shoulder presentation, seen in 1% of births (see Fig. 13-3, D). The presenting part is that part of the fetus that lies closest to the internal os of the cervix. It is the 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. 13-2). Factors that determine the presenting part include fetal lie, fetal attitude, and extension or flexion of the fetal head.

Fetal Lie

Lie is the relation of the long axis (spine) of the fetus to the long axis (spine) of the mother. The two primary lies are longitudinal, or vertical, in which the long axis of the fetus is parallel with the long axis of the mother (see Fig. 13-2); 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. 13-3, 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 (i.e., one in which the long axis of the fetus is lying at an angle to the long axis of the mother) is less common and usually converts to a longitudinal or transverse lie during labor (Cunningham, Leveno, Bloom, et al., 2010).

Fetal Attitude

Attitude is the relation of the fetal body parts to one another. 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 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. 13-2).

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 about 9.25 cm at term, is the largest transverse diameter and an important indicator of fetal head size (Fig. 13-4, B). In a well-flexed cephalic presentation the biparietal diameter is the widest part of the head entering the pelvic inlet. Of the several anteroposterior diameters, the smallest and most critical one is the suboccipitobregmatic diameter (about 9.5 cm at term). When the fetal head is in complete flexion, this diameter allows it to pass through the true pelvis easily (see Fig. 13-4, A; Fig. 13-5, A). As the head is more extended, the anteroposterior diameter widens, and the head may not be able to enter the true pelvis (see Fig. 13-5, B and C).

Fetal Position

The presentation or presenting part indicates the portion of the fetus that overlies the pelvic inlet. Position is the relationship of a reference point on the presenting part (occiput, sacrum, mentum [chin] or sinciput [deflexed vertex]) to the four quadrants of the mother’s pelvis (see Fig. 13-2). 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 (O 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. 13-2). LSP means that the sacrum is the presenting part and is located in the left posterior quadrant of the maternal pelvis (see Fig. 13-3).

Station is the relationship of the presenting fetal part 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. 13-6). 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 the rate of descent of the fetus during labor can be determined accurately.

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. It often occurs in the weeks just before labor begins in nulliparas and may occur before or during labor in multiparas. Engagement can be determined by abdominal or vaginal examination.

Bony Pelvis

The anatomy of the bony pelvis is described in Chapter 3. The following discussion focuses on the importance of pelvic configurations as they relate to the labor process. (It may be helpful to refer to Figs. 3-4 and 3-5.)

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 (Fig. 13-7, A), and the sacrococcygeal joint (Fig. 13-7, B). The bony pelvis is separated by the brim, or inlet, into two parts: the false and the true pelves. The false pelvis is the part above the brim and plays no part in childbearing. The true pelvis, the 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 with 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; and bounded by the pubic arch anteriorly, the ischial tuberosities laterally, and the tip of the coccyx posteriorly (see Fig. 13-7, B). In the latter part of pregnancy the coccyx is movable unless it has been broken (e.g., in a fall during skiing or skating) and has fused to the sacrum during healing.

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. 13-8) 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 is less accommodating than a rounded wide arch. The method of measurement of the subpubic arch is shown in Fig. 13-9. A summary of obstetric measurements is given in Table 13-1.

TABLE 13-1

OBSTETRIC MEASUREMENTS

PLANE	DIAMETER	MEASUREMENTS
Inlet (Superior Strait)		Length of diagonal conjugate (solid colored line), obstetric conjugate (broken colored line), and true conjugate (blue line)*
Conjugates
Diagonal	12.5-13 cm
Obstetric: measurement that determines whether presenting part can engage or enter superior strait	1.5-2 cm less than diagonal (radiographic)
True (vera) (anteroposterior)	≥11 cm (12.5) (radiographic)
Midplane		Measurement of interspinous diameter*
Transverse diameter (interspinous diameter)	10.5 cm
The midplane of the pelvis normally is its largest plane and the one of greatest diameter
Outlet		Use of Thom’s pelvimeter to measure intertuberous diameter*
Transverse diameter (intertuberous diameter) (biischial)	≥8 cm
The outlet presents the smallest plane of the pelvic canal

^*From Seidel HM, Ball JW, Dains JE, et al: Mosby’s guide to physical examination, ed 7, St Louis, 2011, Mosby.

Stay updated, free articles. Join our Telegram channel

Join