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Birth is an ordeal for women everywhere, according to a review of birthing patterns in nearly 300 cultures around the world by Rosenberg and colleague Wenda Trevathan, an anthropologist at New Mexico State University. "Not only is labor difficult," Rosenberg says, "but because of the design of the female pelvis, infants exit the birth canal with the back of their heads against the pubic bones, facing in the opposite direction from the mother. This makes it tough for her to reach down and guide the baby as it emerges without damaging its spine—and also inhibits her ability to clear the baby's breathing passage or to remove the umbilical cord from around its neck. That's why women everywhere seek assistance during labor and delivery."

Compared with humans, most primates have an easier time, Rosenberg says. A baby chimpanzee, for instance, is born quickly: entering, passing through, and leaving its mother's pelvis in a straight shot and emerging faceup so that its mother can pull it forward and lift it toward her breast. In chimps and other primates, the oval birth canal is oriented the same way from beginning to end. In humans, it’s a flattened oval one way and then it shifts orientation 90 degrees so that it’s flattened the other way. To get through, the infant’s head and shoulders have to align with that shifting oval. It’s this changing cross-sectional shape of the passageway that makes human birth difficult and risky, Rosenberg says, not just for babies but also for mothers. A hundred years ago, childbirth was a leading cause of death for women of childbearing age.

Why do we possess a birth canal of such Byzantine design? "The human female pelvis is a classic example of evolutionary compromise," Rosenberg answers. Its design reflects a trade-off between the demand for a skeletal structure that allows for habitual walking on two feet and one that permits the passage of a baby with a big brain and wide shoulders. Its unique features didn't come about all at once, but at different times in our evolutionary history, in response to different selective pressures. "The result of these different pressures is a jerry-rigged, unsatisfactory structure," Rosenberg says. "It works, but only marginally. It's definitely not the type of system you would invent if you were designing it. But evolution is clearly a tinkerer, not an engineer; it has to work with yesterday's model."

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