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. 9-1, A). These bones are united by membranous sutures: the sagittal, lambdoidal, coronal, and frontal (Fig. 9-1, 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 ones (see Fig. 9-1, B). The larger of these, the anterior fontanel, is diamond shaped, is approximately 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 approximately 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. 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 after birth.

Although the size of the fetal shoulders may affect passage, their position can be altered relatively easily during labor; thus one shoulder may occupy a lower level than the other. This position 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

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. 9-2); breech presentation (buttocks or feet first), occurring in 3% of births (Fig. 9-3, A-C); and shoulder presentation, seen in 1% of births (Fig. 9-3, D). The presenting part is 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, and in the shoulder presentation it is the scapula. When the presenting part is the occiput, the presentation is noted as vertex (see Fig. 9-2). Factors that determine the presenting part include fetal lie, fetal attitude, and extension or flexion of the fetal head.

Fetal position

The presentation or presenting part indicates the 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. 9-2). 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. 9-2). LSP means that the sacrum is the presenting part and is located in the left posterior quadrant of the maternal pelvis (see Fig. 9-3).

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. 9-6). 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 said to be 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 nulliparas and may occur before labor 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 2. The following discussion focuses on the importance of pelvic configurations as they relate to the labor process. (Referring to Fig. 2-4 may be helpful.)

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. 9-7, 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 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, bounded by the pubic arch anteriorly, the ischial tuberosities laterally, and the tip of the coccyx posteriorly (Fig. 9-7, 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).

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. 9-8), 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. 9-9. A summary of obstetric measurements is given in Table 9-1.

TABLE 9-1

Obstetric Measurements

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

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