Military Obstetrics & Gynecology
Mechanism of Normal Labor

Mechanism of Normal Labor
There are five classical steps in the normal mechanism of labor. They are:

Descent
Flexion
Internal Rotation
Extension
External Rotation

Usually, labor progresses in this fashion, if the fetus is of average size, with a normally positioned head, in a normal labor pattern in a woman whose pelvis is of average size and gynecoid in shape.

There is overlap of these mechanisms. The fetal head, for example, may continue to flex or increase its flexion while it is also internally rotating and descending.

		Descent: As the fetal head engages and descends, it assumes an occiput transverse position because that is the widest pelvic diameter available for the widest part of the fetal head.
		Flexion: While descending through the pelvis, the fetal head flexes so that the fetal chin is touching the fetal chest. This functionally creates a smaller structure to pass through the maternal pelvis. When flexion occurs, the occipital (posterior) fontanel slides into the center of the birth canal and the anterior fontanel becomes more remote and difficult to feel. The fetal position remains occiput transverse.
		Internal Rotation: With further descent, the occiput rotates anteriorly and the fetal head assumes an oblique orientation. In some cases, the head may rotate completely to the occiput anterior position.
		Extension: The curve of the hollow of the sacrum favors extension of the fetal head as further descent occurs. This means that the fetal chin is no longer touching the fetal chest.
		External Rotation: The shoulders rotate into an oblique or frankly anterior-posterior orientation with further descent. This encourages the fetal head to return to its transverse position. This is also known as restitution.