Obstetrics and Gynecology - A.M. Hromova 2000

Physiology of Childbirth
Biomechanism of Labor in Flexion Presentations
Occiput Posterior Position

The occiput posterior position is a variation of the primary mechanism of labor. When the occiput is directed posteriorly, in the majority of cases the fetal HEAD describes a 135° arc, rotating beneath the pubic arch with the occiput moving anteriorly. Consequently, the sagittal suture—initially situated in one of the oblique diameters of the pelvis, depending on the fetal position (left oblique for the first position, right oblique for the second)—transitions first into the transverse diameter, then into the opposite oblique diameter, and finally into the direct (anteroposterior) diameter. If anterior Rotation of the occiput fails to occur, labor proceeds in the persistent occiput posterior position.

First movement: flexion and descent of the fetal head. A pronounced flexion of the head is observed from the very beginning. The leading point is located midway between the anterior and posterior fontanelles. The sagittal suture engages in the transverse or slightly oblique diameter.

Second movement: internal rotation of the fetal head. Describing a small 45° arc, the sagittal suture enters the anteroposterior diameter of the pelvic outlet; the occiput, and consequently the posterior fontanelle, rotates toward the sacrum, while the anterior fontanelle faces the Pubic Symphysis.

Third movement: additional flexion of the head. The fetal head abuts the lower margin of the symphysis pubis with the area just anterior to the hairline, which serves as the fulcrum (point of fixation). It undergoes further flexion, allowing the parietal eminences and the occipital portion of the head to be born.

Fourth movement: extension of the head occurs As a result of forming a new fulcrum between the suboccipital fossa and the tip of the sacrum. The head is delivered via moderate extension, with the anterior margin of the bregma, forehead, and face emerging from beneath the symphysis. Thus, the head is born facing anteriorly, utilizing the middle oblique diameter of 10 cm and a circumference of 33 cm.

Fifth movement: internal rotation of the trunk and external rotation of the head proceed identically to those in the Anterior Occiput Presentation. The cranial configuration in the occiput posterior position is dolichocephalic.