Medical progress has changed childbirth – potentially enough to have an impact on human evolution
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Women’s pelvises have become narrower in the past 150 years, according to a study of more than 8,000 people from three countries. There are many factors at stake, but whatever the ultimate cause, it is the last proof that researchers lead the researchers to rethink the “obstetrical dilemma”, a description of the evolutionary pressures at war on the size of the basin: the need to adapt to the large heads of babies pushes the pelvises to widen, but the need to walk to the pushing them.
We do not know exactly what stimulates this change, nor all the ways of which it will affect the health of people. But if the pelvises continue to shrink at this rate, it could make the Césarean sections more likely – which could have a multitude of training effects.
Maciej Henneberg at the University of Adélaide in Australia and her colleagues reanalyzed an existing set of data of 1247 Australian women, born between 1900 and 1984, and found that the pelvic width decreased by 0.42 millimeters per year. Similarly, among 3,486 Polish women, basin widths decreased by 0.47 mm per year between 1880 and 1970, and among 320, the width of the Mexican women’s basin decreased by 0.42 mm per year between 1900 and 1970. At the same time, the average height increased and the width of the shoulders, or augmented.
“Given that in these different regions, it has evolved in the same direction, even if the height of the body has increased, I personally find it convincing,” explains Philipp Mitteroecker at the University of Vienna in Austria.
“The data set is fantastic,” explains Lia Betti at the University College in London.
For Henneberg, this observation shows that modern medicine weakens the impact of natural selection on the human basin – and especially on the width of the birth channel. In the past, if a baby was too large or if the birth channel was too narrow, the mother and the baby would probably die in delivery. However, safe and effective cesarean cuts, or cesarean sections – in which the baby is surgically removed through the abdomen – means that this evolutionary pressure is reduced. Consequently, birth channels and pelvises can become narrower.
Mitteroecker also thinks that cesareans change evolving pressures at work on the human birth channel. He predicted that this could encourage pelvises to shrink in a 2016 analysis, and says that it is the last iteration of a longtime phenomenon.
“The midwife is old,” he said, and “really unique to humans”. Women get help with childbirth, often other women, for hundreds of thousands of years. This cultural practice softens the selection pressure on the basin and the birth channel – our behaviors have therefore affected our own biological evolution. “The cesarean is, in a way, an extreme form of that,” said Mitteroecker.
However, Betti is skeptical about the Caesareans is the main explanation for the recent change in the pelvic width. She stresses that humans have become much larger in the same period of time, but this is probably due to the diet and better health care – not an evolutionary change in our genes.
“We know that the diet can affect the pool,” explains Betti. When nutrition is rare, our development bodies tend to allocate more nutrients to certain organs, including the brain, to the detriment of others. But now we have great nutrition, so our body can have reassigned nutrients. “So we end up with different bodily proportions,” explains Betti. “It is quite possible.”
Finding an explanation of our narrowed pelvises could help us understand why human childbirth is so difficult – which brings us back to the obstetric dilemma. However, the exact nature of the dilemma has been disputed. In a study in 2024, Mitteroecker and his colleagues found that the pelvic floor, and not walking, was probably the main driver to narrowed: wider pelvises increase the already high pressure on the pelvic floor, stimulating the risk of prolapse and incontinence.
Or the two influences could be at work. A study of 31,000 people, published in April, linked wider pelvises to easier births, but also to a slower march and a greater risk of pelvic floor conditions.
There could be even more influences in this dilemma. Betti notes that making competitive sports as a child can also affect the size of the basin and maintains that our pelvises are sensitive to many factors in the environment, such as temperature. Other researchers have described an “new obstetrical dilemma” linked to the increase in obesity rates, which can enlarge babies. The real answer is probably a combination of factors, says Betti: she says that some researchers have renamed the dilemma as a “multifactor basin”.
Closer pelvises will affect human health. They will make vaginal childbirth more difficult, potentially leading to even more cesareans. “Who knows how long it takes to reach the state that there will be no children born naturally?” Said Renata Henneberg, who is part of the team behind the new research and the wife of Maciej Henneberg.
At the same time, narrower pelvises can reduce the risk of pelvic floor problems, which can be very harmful. Birthday, says Betti, can have “very unpleasant and lasting effects, which can very negatively affect a woman’s life”.
However, she says that predicting what’s going on is difficult, once again because many factors are at stake: people have fewer children, which could reduce the risk of injury, but they also have them later. “Many things have changed at the same time,” she says.
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