Why can textbooks need to update what they say about birth channels

Look for the term "pelvic canal" in the typical textbook of anatomy or obstetrics, and you will probably find a description like this: "Well-built and healthy women, who have had a good diet during their growth, usually have a large pelvis. "

Such a basin, continues the text, allows "the least difficulty during childbirth".

But these characterizations have long been based on anatomical studies of people of European descent. In fact, the structure of the pelvic canal, the bone structure through which most of us enter the world, varies enormously from one population to another, according to a new study published in Proceedings of the Royal Society B.

The findings have implications for how obstetricians treat patients with color, say the authors. In the United States, for example, the risk of death from pregnancy is three to four times higher in black women than in white women.

"What worries me is that doctors are coming out of school thinking about the European model of the pelvis," said Lia Betti, an anthropologist at the University of Roehampton in London, and lead author of the study. "In predominantly white societies, I imagine that minorities are at greater risk."

Modern humans have a narrow pelvis in relation to the size of the babies' heads. This difference contributes to higher rates of complications at birth in humans than in other primates.

Factors such as the time it takes for a baby to progress through the canal or the direction of the baby's head at the time of delivery may change depending on the shape of the pelvis. These factors, in turn, could influence decisions about when to induce labor, how to help with forceps or at the time of cesarean delivery, Betti said.

There is no accepted explanation for why the human pelvis leaves so little room for childbirth. Betti and his colleague Andrea Manica, of Cambridge University, set out to study a classic, albeit highly contested, explanation known as the "obstetric dilemma" hypothesis.

The dilemma postulates that when our species evolved and started to walk upright, the width of the human pelvis narrowed, allowing the body weight to stay closer to its center of gravity. But as humans also developed larger brains, it was increasingly difficult for the skull of a fetus to sneak through this narrow channel.

Betti is skeptical about this explanation and thinks that other possibilities, such as modern diets or the need to support internal organs, might help explain the inadequacy between the pelvis and the fetus.