Position is the relationship of the presenting part (occiput, sacrum, mentum, or sinciput ) to the four quadrants of the mother’s pelvis (see Fig. The presentation or presenting part indicates the portion of the fetus that overlies the pelvic inlet. As the head is more extended, the anteroposterior diameter widens, and the head may not be able to enter the true pelvis (see Fig. When the head is in complete flexion, this diameter allows the fetal head to pass through the true pelvis easily ( Fig. Of the several anteroposterior diameters, the smallest and the most critical one is the suboccipitobregmatic diameter (approximately 9.5 cm at term). In a well-flexed cephalic presentation, the biparietal diameter will be the widest part of the head entering the pelvic inlet. 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. 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.Ĭertain critical diameters of the fetal head are usually measured. The circumference of the fetal hips is usually small enough not to create problems.ĭeviations from the normal attitude may cause difficulties in childbirth. This position creates a shoulder diameter that is smaller than the skull, facilitating passage through the birth canal. Molding can be extensive, but the heads of most newborns assume their normal shape within 3 days after birth.Īlthough 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 capacity of the bones to slide over one another also permits adaptation to the various diameters of the maternal pelvis. Because the bones are not firmly united, however, slight overlapping of the bones, or molding of the shape of the head, occurs during labor. Sutures and fontanels make the skull flexible to accommodate the infant brain, which continues to grow for some time 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. 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. The two most important fontanels are the anterior and posterior ones (see Fig.
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