Birth canals are different all over the world, countering a long-held evolutionary theory

The shape of a mother’s birth canal is a tug-of-war between two opposing evolutionary forces: It needs to be wide enough to allow our big-brained babies to pass through, yet narrow enough to allow women to walk efficiently. At least that’s been the common thinking. But a new study reveals birth canals come in a variety of shapes in women around the world.

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. The birth canals were far from carbon copies of each other. 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.

Temperature could also be a factor. Colder climates favor wider bodies, which are better at holding in heat, and this could have an impact on birth canal shape. But the pelvic data only weakly followed that trend. Wells argues that other environmental factors may play a role and should still be explored.

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.

The new findings suggest that if a baby’s movements differ from what’s considered normal for a particular region, she says, it’s not necessarily cause for concern. It may simply reflect the range of birth canal shapes seen throughout the world.