Obstetrics and Gynecology - A. M. Gromova 2000

Anomalies of the Bony Pelvis
Course of Pregnancy and Childbirth in Contracted Pelvis
Features of the Biomechanism of Labor in Generally Uniformly Contracted Pelvis

A distinctive feature of the labor mechanism in a generally evenly contracted pelvis is that the fetal HEAD, adapting for a prolonged period to the reduced pelvic dimensions to facilitate engagement, undergoes maximum flexion. As a result, it descends into the pelvic cavity via its smallest dimension—the dist. suboccipitoparietalis, which corresponds to the distance from the midpoint of the sagittal suture between the two fontanelles to the suboccipital fossa (this being even smaller than the suboccipito-bregmatic diameter). The sagittal suture lies in one of the oblique diameters, which prove to be the longest in this type of contraction. With such flexion, the small fontanelle is located close to the guiding point, and the closer it gets to it, the greater the cephalopelvic disproportion.

The head maintains this pronounced flexion within the pelvic cavity as well. Internal rotation is correct (occiput anterior), but significantly slower. Constricted on all sides in the direction of the suboccipito-bregmatic and transverse diameters, the head is elongated into a wedge-like shape with a caput succedaneum forming over the area of the small fontanelle, acquiring a distinct dolichocephalic shape. The caput succedaneum is sometimes so large that it reaches the pelvic floor while the head itself is still positioned in the upper part of the pelvic cavity. This must be taken into account during the examination. Having reached the pelvic outlet with the sagittal suture in the direct (anteroposterior) diameter, the head begins to extend, and further labor proceeds as in a normal pelvis; however, a severe perineal tear is possible (the suboccipital fossa does not fit snugly against the pubic arch, but only partially touches it due to the narrowed pubic angle, causing the head to sweep rapidly toward the Perineum, overstretching it and causing a rupture).

Nevertheless, all Stages of the labor mechanism and the overall duration of the birthing process in a generally evenly contracted pelvis are prolonged and require sustained, vigorous uterine activity for completion.



Last update: 08/08/2026

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