Evolutionary obstetrics

Birth is a challenging and perhaps painful process for primates that have large heads relative to their body size. This includes our own species, but for humans, the pelvic changes resulting from the origin of bipedalism5–7millionyearsagorestructured the pelvis, altered the birth mechanism and further limited the size of the birth canal. Additional challenges to the birth process occurred with dramatically increased brain size starting about 2 million years ago [1]. The bipedal birth canal is ‘twisted’ in the middle with the inlet and outlet perpendicular. In between these two planes is the midplane, which usually presents the smallest diameter. The changing shape typically involvesrotationsof theinfant inorder to pass through the three pelvic planes to emerge with the occiput anterior (OA), facing away from the mother. These challenges place a selective advantage on having assistance for guiding the baby out, clearing the face and nose and dealing withawrapped umbilical cord.Worldwide it is rare for women to give birth alone. Western medical perspectives on childbirth view it as sufficiently risky that it must take place in a clinical setting. Examples in human biology and public health