Because humans are bipedal with an erect stance and have, in relation to the size of the pelvis, the biggest head of any mammalian species, human fetuses and human female pelvises are adapted to make birth possible. The erect posture causes the weight of the abdominal contents to thrust on the pelvic floor, a complex structure which must not only support this weight but allow three channels to pass through it: the urethra, the vagina and the rectum. The head and shoulders require a specific sequence of maneuvers to occur for the bony head and shoulders to pass through the bony ring of the pelvis Six phases of a typical vertex (head-first presentation) delivery: 1. Engagement of the fetal head in the transverse position. The baby's head is facing across the pelvis at one or other of the mother's hips. 2. Descent and flexion of the fetal head. 3. Internal rotation. The fetal head rotates 90 degrees to the occipito-anterior position so that the baby's face is towards the mother's rectum. 4. Delivery by extension. The fetal head passes out of the birth canal. Its head is tilted forwards so that the crown of its head leads the way through the vagina.
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