Obstetrics and Gynecology - A.M. Hromova 2000

Physiology of Childbirth
Biomechanism of Labor in Flexion Presentations

The biomechanism of labor is a predictable sequence of rotational and translational movements performed by the fetus as it advances through the birth canal.

By the end of Pregnancy, the fetus typically assumes a longitudinal lie. In 95% of cases, this is a cephalic presentation, while breech presentation occurs in 3–3.5% of cases; transverse or oblique lies are rare.

The fetal HEAD is in a state of moderate flexion. Its sagittal suture is equidistant from the Pubic Symphysis and the sacral promontory (synclitic engagement). The chin is tucked toward the chest, the back is curved, the arms are crossed over the chest, and the legs are flexed at the knees and hips and drawn up toward the abdomen. By the end of pregnancy, the presenting part is typically positioned as close as possible to the pelvic inlet or slightly engaged within it.



Last update: 08/08/2026

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