When the fetus is of average size, with a normally positioned head, 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.
The widest diameter of the presenting part (with a well-flexed head, where the largest transverse diameter of the fetal occiput is the biparietal diameter) enters the maternal pelvis to a level below the plane of the pelvic inlet. On the pelvic examination, the presenting part is at 0 station, or at the level of the maternal ischial spines.
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.
While descending through the pelvis, the fetal head flexes so that the fetal chin is touching the fetal chest so that a smaller structure passes through the maternal pelvis. When flexion occurs, the occipital (posterior) fontanel slides into the center of the birth canal and the anterior fontanel becomes is difficult to feel. The fetal position remains occiput transverse.
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.
The curve of the hollow of the sacrum favors extension of the fetal head as further descent occurs. The fetal chin is no longer touching the fetal chest.
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.
After the fetus' head is delivered, further descent brings the anterior shoulder to the level of the pubic symphysis. The anterior shoulder is then rotated under the symphysis, followed by the posterior shoulder and the rest of the fetus.