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Why Is Human Childbirth So Painful?

Having babies isn’t easy—and the standard explanation may be wrong.

Pat Shipman

Casting Doubt on a Paradigm

2013-11MargShipmanF2.jpgClick to Enlarge ImageThe obstetrical hypothesis postulates that the demands of an unusual locomotor system increase the risk and cost of the reproductive process. If this is the case, evolution would favor human birth at earlier stages of development than in other, nonbipedal primates, and mothers with wider hips would experience decreased motor efficiency.

The obstetrical hypothesis is neat and readily comprehended, which helps explain its widespread acceptance, but new evidence casts doubt on it. A recent paper by Holly Dunsworth of the University of Rhode Island and colleagues reexamines the predictions and evidence supporting the obstetrical hypothesis and suggests an alternative explanation. For instance, human gestation is often said to be short relative to that of other primates, based on how much more growth is needed in neonates to achieve adult brain size. The shorter duration of gestation on first glance supports a prediction of the obstetrical hypothesis—that birth has evolved to occur earlier in hominids so that the baby is born before its head is too large to pass through the birth canal. Actually, the duration of human pregnancy (38–40 weeks) is absolutely longer than that of chimps, gorillas, and orangutans (32 weeks for chimps and 37–38 weeks for the latter two). When Dunsworth and her colleagues took maternal body size into account, which in primates is positively correlated with gestation length, they showed that human pregnancy is also relatively longer compared to that in great apes. No wonder that the third trimester seems so long to many pregnant women.

Another oft-cited fact supporting the obstetrical hypothesis is that, of all the primates, human newborns have the least-developed brains. Human babies’ brains are only 30 percent of adult size, as opposed to 40 percent in chimps. This difference in newborn brain size seems to suggest that human babies are born at an earlier developmental stage than other primates.

2013-11MargShipmanF3.jpgClick to Enlarge ImageThe catch is that adult brain size in humans is much larger than in other primates for reasons having nothing to do with birth. This means that using adult brain size as a basis for comparing relative gestation length or newborn brain size among primates will underestimate human development. But as one of the collaborators with Dunsworth, Peter Ellison of Harvard University, pointed out in his 2001 book Fertile Ground, the relevant question is, Given how large a mother’s body size is, how big a brain can she afford to grow in her baby? It is an issue of supply and demand. Labor occurs when the mother can no longer continue to supply the baby’s nutritional and metabolic demands.

As Ellison puts it, “Birth occurs when the fetus starts to starve.” From this perspective, the brain size of a human newborn is not small for a primate but is very large—one standard deviation above the mean. Body size in human newborns is also large relative to other primates when standardized for a mother’s body size. Both facts suggest that pregnancy may push human mothers to their metabolic limits.

The obstetrical hypothesis, in contrast, suggests that locomotion rather than metabolism is the limiting factor in birth size. The underlying concept here is that wider-hipped women—capable of giving birth to larger offspring—should suffer a disadvantage in locomotion. But detailed studies of the cost of running and walking—including new work by Dunsworth’s coauthors Anna G. Warrener of Harvard University and Herman Pontzer of Hunter College—do not support this idea. Men and women are extremely similar in the cost and efficiency of locomotion, regardless of hip width. Enlarging the birth canal to pass a baby with a brain 40 percent of adult size, as is typical of newborn chimps, would require an increase in diameter of only three centimeters—just over an inch—in the smallest dimension of the birth canal. This wouldn’t hinder locomotion significantly, given that many women already have such broad hips. The conflict between big-brained babies and upright walking may be more conceptual than real.

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