Birth Canals Are Different All over the World, Countering a Long-Held Evolutionary Theory

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The idea that women’s pelvises have been shaped by an evolutionary compromise — also known as the “obstetrical dilemma” — has been influential in anthropology, says Jonathan Wells, an expert in human evolution at University College London who was not involved with the work. But recent studies have challenged it, and the new findings add to that research, he says. If the obstetric dilemma held true, one would expect birth canals around the world to be relatively standardized, Wells says. But that’s not what researchers found.

Lia Betti, a biological anthropologist at the University of Roehampton in London and evolutionary ecologist Andrea Manica of the University of Cambridge in the United Kingdom, measured the pelvises of 348 female human skeletons from 24 different parts of the world. {snip} Those of women from sub-Saharan Africa and some Asian populations were overall narrow from side to side and deep from front to back, whereas Native American women had wider canals. Native Americans and Europeans also had the most oval-shaped upper canals, the team reports today in the Proceedings of the Royal Society B.

Betti and Manica also found that there was less variability in birth canal shape in populations farther from Africa, such as Native Americans. That pattern has been seen in other traits, and is thought to simply reflect lower variability in genes and traits among the relatively small bands of people who moved out of Africa to populate the world. Overall, the analysis suggests a population may have ended up with a particular birth canal shape simply by chance, not because of any sort of selective pressure.

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The work could improve practices surrounding childbirth, Betti says. For example, a fetus must rotate to negotiate the twisting passage of the birth canal during labor, and these movements may vary depending on the shape of the birth canal. Betti says midwives she has talked to are well aware that women from different parts of the world have marked differences in labor, though it’s not part of their formal training.

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[Editor’s Note: The complete Beti-Manica study is available here.]