Obstetrics and Gynecology - A.M. Gromova 2000

Physiology of Labor
Biomechanism of Labor in Flexion Presentations
Theories of the Biomechanism of Labor

There are many theories capable of explaining the biomechanism of labor to a certain extent, yet none of them can fully account for its causative nature. According to A. Ya. Krasovsky, the flexion of the fetal HEAD is driven by the pressure exerted by the spine, which transmits the action of expulsive forces. The head is thus viewed as an unequal-armed lever because the junction between the spine and the Base of the Skull is located closer to the occiput than to the chin rather than at the center. Consequently, the greater portion of the expulsive forces concentrates on the occipital region. The longer lever arm (the face and forehead) encounters resistance from the linea innominata of the pelvis. This generates two opposing forces: one presses upon the occiput, driving it forward, while the other restrains the fetal face. As a result, the occiput descends lower than the forehead, leading to flexion of the head.

Internal Rotation of the head is explained by the interaction between the fetal head, the pelvic walls, and the Muscles of the pelvic floor. Gliding along the pelvic walls and influenced by the pelvic floor musculature, the head performs its internal rotation. Following this internal rotation, the head remains in a state of marked flexion, positioning its occipital region between the inferior rami of the pubic bones. Here, the occiput experiences pressure from the expulsive forces and emerges from beneath the pubic arch first. Subsequently, as a result of the interplay between two intersecting forces—the force generated by the action of the abdominal wall and Diaphragm along with the uterine muscles, and the counterforce of the pelvic floor muscles—the head undergoes extension.

Internal rotation of the trunk and external rotation of the head are driven primarily by the rotation of the shoulder girdle.