In most occiput posterior presentations, the mechanism of labor is identical to that observed in the transverse and anterior varieties, except that the occiput has to internally rotate to the symphysis pubis through 135 degrees, instead of 90 and 45 degrees, respectively (see Fig. 22-15).
Efective contractions, adequate head lexion, and average fetal size together permit most posteriorly positioned occiputs to rotate promptly as soon as they reach the pelvic loor, and labor is not lengthened appreciably. In perhaps 5 to 10 percent of cases, however, rotation may be incomplete or may not take place at all, especially if the fetus is large (Gardberg, 1994b). Poor contractions, faulty head lexion, or epidural analgesia, which diminishes abdominal muscular pushing and relaxes pelvic floor muscles, may predispose to incomplete rotation. If rotation is incomplete, transverse arrest may result. If no rotation toward the symphysis takes place, the occiput may remain in the direct occiput posterior position, a condition known as persistent occiput posterior. Both can lead to dystocia and cesar ean delivery. Techniques to manually rotate from OP to OA positions are illustrated in Chapter 29 (p. 560).
In vertex presentations, labor forces alter fetal head shape. In prolonged labors before complete cervical dilation, the portion of the fetal scalp immediately over the cervical os becomes edematous. his swelling is known as the caput succedaneum (rig. 22-16). It usually attains a.thickness of only a few millimeters, but in prolonged labors it may be suiciently extensive
FIGURE 22-16 Considerable molding of the head and caput succedaneum formation in a recently delivered newborn.
to prevent diferentiation of the various sutures and fontanels. More commonly, the caput is formed when the head is in the lower portion of the birth canal and frequently only after the resistance of a rigid vaginal outlet is encountered. Because it develops over the most dependent area of the head, one may deduce the original fetal head position by noting the location of the caput succedaneum.
Molding refers to changes in the bony fetal head shape as a result of external compressive forces (see Fig. 22-16). Possibly related to Braxton Hicks contractions, some molding develops before labor. Most studies indicate that there is seldom overlapping of the parietal bones. A "locking" mechanism at the coronal and lambdoidal sutures actually prevents such overlapping (Carlan, 1991). Molding results in a shortened suboccipitobregmatic diameter and a lengthened mentovertical diameter. These changes are of greatest importance in women with contracted pelves or asynclitic presentations. In these circumstances, the degree to which the head is capable of molding may make the diference between spontaneous vaginal delivery and an operative delivery. Some older literature cited severe head molding as a cause for possible cerebral trauma. Because of the multitude of associated factors, for example, prolonged labor with fetal sepsis and acidosis, it is impossible to link molding to any alleged fetal or neonatal neurological sequelae. .10st cases of molding resolve within the week following delivery, although persistent cases have been described (Graham, 2006). Diferentiation of molding, caput succedaneum, and cephalohematoma is discussed in Chapter 33 (p. 628).
The greatest impediment to understanding normal labor is recognizing its start. he strict deinition oflabor is: uterine contractions that bring about demonstrable facement and dilation of the cervix. This does not easily aid the clinician in determining when labor has actually begun, because this diagnosis is conirmed only retrospectively. Several methods may be used to mark its start. One defines onset as the clock time when painful contractions become regular. Unfortunately, uterine activity that causes discomfort, but that does not represent true labor, may develop