Why Is Human Childbirth So Painful?
Having babies isn’t easy—and the standard explanation may be wrong.
What Does a Baby Cost?
Although 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.