A small woman is likely to have a small pelvis, but she is also likely to have a small neonate. Thoms (1937) studied 362 nulliparas and found that the mean birth weight of their ofspring was significantly lower-280 g-in women with a small pelvis than in those with a medium or large pelvis.
Normally, cervical dilation is aided by hydrostatic action of the unruptured membranes or after their rupture, by direct application of the presenting part against the cervix. In con tracted pelves, however, because the head is arrested in the pelvic inlet, the entire force exerted by the uterus acts directly on the portion of membranes that contact the dilating cervix.
Consequently, early spontaneous rupture of the membranes is more likely.
After membrane rupture, absent pressure by the head against the cervix and lower uterine segment predisposes to less efec tive contractions. Hence, further dilation may proceed very slowly or not at all. Cibils and Hendricks (1965) reported that the mechanical adaptation of the fetal passenger to the bony passage plays an important part in determining the eiciency of contractions. The better the adaptation, the more eicient the contractions. hus, cervical response to labor provides a prog nostic view of labor outcome in women with inlet contraction.
A contracted inlet also plays an important part in the pro duction of abnormal presentations. In nulliparas with normal pelvic capacity, the presenting part at term commonly descends into the pelvic cavity before labor onset. When the inlet is con tracted considerably or there is marked asynclitism, descent usually does not take place until after labor onset, if at all.
Cephalic presentations still predominate, but the head loats freely over the pelvic inlet or rests more laterally in one of the iliac fossae. Accordingly, very slight influences may cause the fetus to assume other presentations. In women with contracted pelves, face and shoulder presentations are encountered three times more frequently, and the cord prolapses four to six times more often.
his inding is more common than inlet contraction. It frequently causes transverse arrest of the fetal head, which potentially can lead to a diicult midforceps operation or to cesarean delivery.
The obstetrical plane of the midpelvis extends from the inferior margin of the symphysis pubis through the ischial spines and touches the sacrum near the junction of the fourth and ifth vertebrae. A transverse line theoretically connecting the ischial spines divides the midpelvis into anterior and posterior portions (Fig. 2-16, p. 30). he former is bounded anteriorly by the lower border of the symphysis pubis and laterally by the ischiopubic rami. The posterior portion is bounded dorsally by the sacrum and laterally by the sacrospinous ligaments, forming the lower limits of the sacrosciatic notch.
Average midpelvis measurements are as follows: transverse, or interischial spinous, 10.5 cm; anteroposterior, from the lower border of the symphysis pubis to the junction of 54-5, 11.5 cm; and posterior sagittal, from the midpoint of the interspinous line to the same point on the sacrum, 5 cm. he deinition of midpelvic contractions has not been established with the same precision possible for inlet contractions. Even so, the midpelvis is likely contracted when the sum of the interspinous and posterior sagittal diameters of the midpelvis-normally, 10.5 plus 5 cm, or 15.5 cm-falls to 13.5 cm or less. his concept was emphasized by Chen and Huang (1982) in evaluating possible midpelvic contraction. Nlidpelvic contraction is suspected whenever the interspinous diameter is < 10 cm. When it mea sures < 8 cm, the midpelvis is contracted.