FIGURE 22-13 Lever action produces flexion of the head. Conversion from occipitofrontal (left) to suboccipitobregmatic (right) diameter typically reduces the anteroposterior diameter from nearly 12 to 9.5 cm.
toward the hollow of the sacrum (Figs. 22-14 and 22-15).
Internal rotation is essential for completion of labor, except when the fetus is unusually small.
Calkins (1939) studied more than 5000 women in labor to ascertain the time of internal rotation. He concluded that in approximately two thirds, internal rotation is completed by the time the head reaches the pelvic loor; in about another fourth, internal rotation is completed shortly after the head reaches the pelvic loor; and in the remaining 5 percent, rotation does not take place. When the head fails to turn until reaching the pelvic floor, it typically rotates during the next one or two contrac tions in multiparas. In nulliparas, rotation usually occurs dur ing the next three to ive contractions.
Ater internal rotation, the sharply flexed head reaches the vulva and undergoes extension. If the sharply flexed head, on reaching the pelvic floor, did not extend but was driven farther downward, it would impinge on the posterior portion of the perineum and would eventually be forced through the perineal tissues. When the head presses on the pelvic floor, however, two forces come into play. he irst force, exerted by the uterus, acts more posteriorly, and the second, supplied by the resistant pelvic floor and the symphysis, acts more anteriorly. he resultant vector is in the direction of the vulvar opening, thereby causing head extension. his brings the base of the occiput into direct contact with the inferior margin of the symphysis pubis (see Fig. 22-14).
With progressive distention of the perineum and vaginal opening, an increasingly large portion of the occiput gradually appears. The head is born as the occiput, bregma, forehead, nose, mouth, and inally the chin pass successively over the anterior margin of the perineum. Immediately after its delivery, the head drops downward so that the chin lies over the maternal anus.
he delivered head next undergoes restitution (see Fig. 22-11). If the occiput was originally directed toward the left, it rotates
FIGURE 22-14 Mechanism of labor for the left occiput transverse position, lateral view. A. Engagement with posterior asynclitism at the pelvic brim. During descent, the sagittal suture is then deflected toward the sacrum. B. This leads to anterior asynclitism. C. Internal rotation and descent. D. Further internal rotation and descent with extension of the neck.
FIGURE 22-15 Mechanism of labor for right occiput posterior position showing anterior rotation.
toward the left ischial tuberosity. If it was originally directed toward the right, the occiput rotates to the right. Restitution of the head to the oblique position is followed by external rotation completion to again reach a transverse position. This movement corresponds to rotation of the fetal body and serves to bring its bisacromial diameter into relation with the anteroposterior diameter of the pelvic outlet. hus, one shoulder is anterior behind the symphysis and the other is posterior. his movement apparently is brought about by the same pelvic factors that produced internal rotation of the head.
lmost immediately after external rotation, the anterior shoulder appears under the symphysis pubis, and the perineum soon becomes distended by the posterior shoulder. After delivery of the shoulders, the rest of the body quickly passes. When the anterior shoulder is tightly wedged beneath the symphysis, then shoulder dystocia is diagnosed, which is described in Chapter 27 (p. 520).
In approximately 20 percent of labors, the fetus enters the pelvis in an occiput posterior (OP) position (Caldwell, 1934). The right occiput posterior (RaP) is slightly more common than the left (LOP). It appears likely from radiographic evidence that posterior positions are more often associated with a narrow forepelvis. They also are more commonly seen in association wi th anterior placentation (Gardberg, 1994a).