Birth, Evolutionary Perspective

The Birth Process

Humans differ from nonhuman primates because they walk bipedally (on two legs) rather than on all fours. The change to bipedalism resulted in a dramatic reconfiguration of human locomotor anatomy (including the pelvis, back, leg and foot) as well as of the birth canal, which passes through the pelvis.

In most primates, birth is a challenging event as the large neonatal head fits tightly within the mother’s birth canal, but in humans, the challenges are especially great because the neonate, with a head large even by primate standards, must negotiate the birth canal, which has been modified from the simple straight passage within the pelvis of quadrupedal apes to one that has a changing cross-sectional shape from inlet (widest in the transverse dimension) to midplane (which is most spacious in the front to back dimension) to outlet (approximately round).

Human neonates are very large in overall body size relative to the size of their mothers and have large heads and broad, rigid shoulders. All this means that the human neonate typically rotates twice as it passes through the birth canal (what obstetricians call the cardinal movements of labor). The infant head usually enters the birth canal (at the inlet) facing transversely and then rotates within the birth canal as it descends to emerge with the long axis of its head aligned with the midline of the mother’s body.

Because the front of the birth canal is most spacious and the back of the infant’s head is broadest, it generally emerges facing away from the mother, a position obstetricians call “occiput anterior” (see Figure 1). The shoulders are broad and must pass through the same constrained and twisting passage to follow the same path as the head, so the infant’s head rotates after it has emerged from the mother’s body. Although this is not the only mechanism of labor and delivery in humans, it is by far the most common, and importantly from an evolutionary perspective, it is the one associated with the fewest risks of injury or death to both the infant and the mother.

Figure 1 The Human Birth Process: Lateral View

Source: Rosenberg, K. and Trevathan, W. (1996). Bipedalism and human birth: The obstetrical dilemma revisited. Evolutionary Anthropology Issues, News, and Reviews 4: 161–168. Reproduced with permission.

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