Among the forms of the narrow pelvis, the uniformly narrowed and simple flat thaw is more common than others . In relation to all forms of the narrow pelvis, these two forms instead make up from 38.1 to 98.9%. Therefore, we dwell on some features of the mechanism of childbirth precisely with these two forms of narrow pelvis, which have the greatest practical significance.

With a general narrowed pelvis, the fetal head is installed, as usual , in one of the oblique dimensions of the entrance to the pelvis in the position of moderate flexion. Experiencing great resistance from the entrance side, it under the influence of strong contractions of the uterus bends more and more and stretches significantly in length. The small fontanel is very low and is located close to the pelvic axis. Often a large head tumor is formed, which can simulate the rapid advancement of the head.
In such cases, the head tumor is already shown in the genital fissure, and the base of the skull and chin are determined above the entrance to the pelvis.
Flexion and internal rotation of the head in a narrowed pelvis require considerable time and good rhythmic labor. The extremely pronounced configuration of the head sometimes leads to the fact that at birth it has a shape similar to a cucumber. With a general narrowed pelvis, the pubic arch approaches in shape to an acute angle, as a result of which the fetal head deviates posteriorly. This delays delivery even more and causes extensive ruptures of the perineum and posterior wall of the vagina. Nevertheless, with a uniformly narrowed pelvis in 80-90%, childbirth occurs spontaneously, without surgical intervention.
With a flat basin, the mechanism of labor is more complex and is characterized by three features (A. Yu. Lurie).

1. Long standing swept seam in the transverse size of the entrance to the pelvis. At the same time, the more massive occipital part of the head, encountering greater resistance of the pelvic bones, the entrance of which is narrowed in the transverse size, is delayed, and the less massive front part of the head with its bitemporal size drops the rape , resulting in its bent position to some extent .
2. In connection with the foregoing, the large fontanel descends and approaches the conductive axis of the pelvis; the small fontanel is higher than the large one and it is difficult to determine it, and sometimes it is not probed at all, since it lingers above the terminal line of the pelvis.
3. Severe asynclitism . First, the head sinks into the pelvis synclitically , that is, the sagittal suture, standing in the transverse dimension, occupies a middle position between the bosom and the cape. Further, with inconsistency and good labor, the so-called declination of the head occurs, that is, its rotation around the sagittal axis. In this case, if the cape is a greater obstacle (which is more favorable), then the anterior parietal bone is lowered earlier, and in this case the sagittal suture will be located closer to the cape. This insertion of the head is called anteroposterior , or anterior asynclitism . If the pubic articulation (which is less favorable) exerts a stronger opposition to the head, then the posterior posterior bone is lowered into the pelvis , the swept stitch is closer to the bosom; a posterotomial insertion is formed, that is, posterior asynclitism , indicating a mismatch between the dimensions of the pelvis and head.

In accordance with the described mechanism, a characteristic configuration of the head is produced: on the parietal bone that goes in front (depending on the type of asynclitism ), a birth tumor forms, the “lagging” parietal bone is flattened due to the back pressure of the corresponding part of the pelvis (cape or lobe ) and goes under the parietal shaft in front bone – the head is as if beveled sideways. On the “lagging” parietal bone from prolonged pressure, the promontory can sometimes form a deep impression, as a result of which the head is flattened in the transverse dimension and thus adapts to the pelvis flattened in the transverse dimension, that is, to a shortened direct size.

In the future, there are usually no obstacles to the birth of the head. However, with very pronounced degrees of posterior asynclitism , a very dangerous situation is sometimes created for both the fetus and the mother. In these cases, a delayed anterior parietal bone and the corresponding ear of the fetus are probed next to the bosom. The head cannot pass through the pelvis, as, descending, it is directed forward, towards the bosom, and meets an insurmountable obstacle here. The rear shoulder, falling, rests on the cape and makes it more difficult to advance the head. There is a strong overstretching of the posterior semicircle of the lower segment of the uterus, which in the absence of timely obstetric care can lead to rupture of the uterus.