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Why Is Human Childbirth So Painful?


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.

What Does a Baby Cost?

2013-11MargShipmanF4.jpgClick to Enlarge ImageAlthough the findings showing that human babies are not earlier than other primates are interesting, they still fail to identify what limits baby brain size. Dunsworth and her coauthors propose that the metabolic constraints faced by a mother limit the length of pregnancy and fetal growth. They have dubbed their hypothesis the energetics-of-gestation-and-growth hypothesis.

As the baby grows in both brain and body in the womb, its demand for energy accelerates exponentially. At some point, the mother reaches the limit of her ability to supply the fetus’s demands, and then labor begins. Even following birth, the big-brained, big-bodied newborn needs a loving mother who will continue to feed and care for it while its brain continues to grow at a fetal rate. In the womb, the fetus is basically part of the mother. Once born, the baby is effectively at a higher trophic level than its mother, like a parasite feeding on her, which increases the metabolic demands on her. However, the baby’s needs have shifted to include more long-chain fatty acids, which are key for brain growth. Since these are very efficiently transmitted to the baby through breast milk, rather than through the placenta, moving the baby outside the womb isn’t a problem.

The obstetrical hypothesis is not defunct; it is simply under question. But merely convincing those who were raised intellectually within this paradigm to consider an alternative hypothesis can be challenging. When she gives a talk about the energetics hypothesis, Dunsworth summarizes a conversation that illustrates this challenge:

What always comes next is, “then why doesn’t the pelvis get wider to make childbirth easier?” And my answer is always, “Because it’s good enough. Witness over seven billion humans on the planet.” But that doesn’t satisfy most people who are moved to ask the question in the first place. And when they argue “the tight fit at birth is too much of a coincidence to ignore,” I ask, “Isn’t it just a coincidence that my finger fits perfectly into my nostril?”

She’s right. Evolutionary adaptation doesn’t have to be perfect, just good enough. Perhaps the female pelvis adapted to fit the size of the human fetus’s brain, rather than the female pelvis’s limiting the baby’s brain size. Still, we are left with no clear reason why a baby is such a tight fit in the mother’s birth canal. Pelvic size may be limited by something not yet taken into account in locomotor studies, such as speed, balance, or risk of injury. Or, perhaps simple economy keeps pelvic size close to neonatal brain size. The third alternative is that human childbirth was not always difficult and has only become so as improvements in diet have increased newborn body size. The obstetrical hypothesis and the energetics hypothesis are not mutually exclusive.

The evolutionary conflict that makes human birthing difficult may not be between walking or running and having babies, but between the fetus’s metabolic needs and the mother’s ability to meet them. Perhaps the problem isn’t only having—bearing—a big-brained baby. Perhaps the real problem is making one.


  • Albers, L. L. 1999. The duration of labor in healthy women. Journal of Perinatology 23:465–475.
  • Dunsworth, H. M., A. G. Warrener, T. Deacon, P. Ellison, and H. Pontzer. 2012. Metabolic hypothesis for human altriciality. Proceedings of the National Academy of Sciences of the U.S.A. 109:15212–15216.
  • Ellison, P. 2001. On Fertile Ground: A Natural History of Human Reproduction. Cambridge, MA: Harvard University Press.
  • Martin, R. D. 2013. How We Do It: The Evolution and Future of Human Reproduction. New York: Basic Books.
  • Portmann, A. 1969. A Zoologist Looks at Humankind. Basel: Schwabe. English translation 1990, Columbia University Press, NY.
  • Wells, J. C. K., J. M. Desilva, and J. T. Stock. 2012. The obstetric dilemma: An ancient game of Russian roulette, or a variable dilemma sensitive to ecology? Yearbook of Physical Anthropology 149(suppl. 55):40–71.

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