Obstetrics and Gynecology - A.M. Hromova 2000

Anomalies of the Bony Pelvis
Course of Pregnancy and Labor in a Contracted Pelvis
Features of the Biomechanism of Labor in a Flat Rachitic Pelvis

In a flat rachitic pelvis, the fetal HEAD adapts to the reduced direct dimension of the pelvic inlet for a long time, undergoing slight extension (with the large fontanelle positioned lower than the small one). This allows the smallest dimension of the head—the small transverse diameter (8 cm)—to engage in the narrowed pelvic dimension. The sagittal suture aligns with the transverse dimension of the pelvis. The sacral promontory, protruding into the pelvic cavity, holds back the posterior Parietal bone. As a result, the anterior parietal bone engages in the pelvic inlet, leading to anterior asynclitism (Naegele's asynclitism).

In this position, the head remains at the pelvic inlet for a prolonged period because it cannot quickly overcome the resistance caused by the narrowed dimension of the pelvic inlet. Head molding and descent then begin. The posterior parietal bone slips off the sacral promontory and significantly overlaps the anterior one. The transverse diameter of the head decreases until it is able to pass through the bottleneck (the direct dimension of the inlet).

Having overcome the resistance of the sacral promontory, the head descends into the pelvic cavity, where spatial relations are more favorable. The sagittal suture gradually transitions into a synclitic engagement. The subsequent biomechanism of labor proceeds normally, as in occipital presentation. Expulsion of the fetus occurs very rapidly due to the enlarged dimensions of the pelvic outlet typical of a flat rachitic pelvis.

Persistent transverse arrest of the sagittal suture may occur in cases of hyperactive labor (when the head fails to rotate in time), necessitating extraction of the head using a vacuum extractor or the application of atypical forceps. During tumultuous labor, if timely assistance is not provided, intracranial Hemorrhage in the fetus and perineal rupture in the mother may occur due to rapid crowning through the vulvar ring. A pronounced caput succedaneum typically forms on the presenting parietal bone, while an indentation is frequently observed on the opposite bone due to compression against the sacral promontory.

In rarer cases involving a flat rachitic pelvis, the posterior parietal bone enters the pelvic inlet, the sagittal suture moves closer to the pubic bone, and the posterior shoulder rests against the sacral promontory. The anterior parietal bone is held back by the pubic bone, resulting in posterior asynclitism (Litzmann's asynclitism), which generally makes spontaneous labor impossible.