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Jump to Key Points For decades, humans have been considered to have uniquely difficult pregnancies due to the “obstetric dilemma”: the evolutionary trade-off between having big-brained babies and walking upright.
But recent research suggests another factor could be at play, one not unique to humans: Male offspring, which tend to be larger, are more likely to cause birth complications.
An Anthropological Perspective
Past studies have substantiated the theory male offspring lead to slightly elevated risks during pregnancy. But there was a gap in the literature.
“Male sex keeps showing up as a significant predictor of difficulties in birth, but we weren’t seeing that in the archeological record,” said Jonathan Wells, PhD, professor of anthropology and paediatric nutrition at University College London in London, England. “The literature focuses on things you can modify, such as maternal BMI.”
- Male fetuses generally larger; birth-complication risk ↑ vs female fetuses.
- Comparative data: 140 mammal species; male size bias common across mammals.
- Larger male offspring linked to dystocia, CPD, shoulder dystocia, ↑ cesarean need.
- Human boys ≈3% heavier at birth; used as reference for interspecies comparison.
- Population effect modest; one study found fetal sex <1% of overall intervention risk.
Wells and Nicole D.S. Grunstra, PhD, researcher with the Department of Evolutionary Biology at the University of Vienna in Vienna, Austria, who specializes in research on the mammalian pelvis, wanted to find an evolutionary pattern to support the medical trend.
Because human pregnancy is difficult to study using the fossil record, Grunstra said the pair thought looking to other mammals might help. “The power of the comparative approach is that you can kind of go outside of humans and say, if we see already a broader pattern in other species, then that really lends credence to the idea that our ancestors also dealt with these challenges,” she said.
In a recent study published in The Anatomical Record, the researchers studied data from 140 mammal species, including humans. In species in which males are generally larger than humans as fetuses, the risk for mechanical difficulties during delivery, including cesarean section necessity, cephalopelvic disproportion, and shoulder dystocia, was elevated. Without medical interference for these conditions, Grunstra said, the “almost inevitable outcome is the death of the offspring.” Wells and Grunstra call this the “costly son” hypothesis.
The study examined both altricial — typically small — and precocial — typically large — neonatal body types relative to maternal size. In humans, boys are around 3% heavier, which the researchers used as a reference value for other species.
Across the board, male babies are bigger than female babies, which corresponds to a sex-based difference in adult body size. In two thirds of the species, male offspring were larger at birth, and in 78% of species with larger adult males, males were at least 3% heavier at birth than females.
Sex Risk During Childbirth
In previous research, Wells found mothers in Nepal giving birth to male babies had an 18% increased chance of experiencing obstructed labor.
Martin Häusler, MD, PhD, published a 2026 study months before in The Anatomical Record, which found the sex of the baby had what he called a “surprisingly small” effect on the risk of requiring interventions such as a cesarean section and the use of forceps — accounting for less than 1% of the overall risk.
Häusler, head of the Evolutionary Morphology and Adaptation Group at the Institute of Evolutionary Medicine at the University of Zürich in Zürich, Switzerland, agreed fetal sex was a factor in predicting if a pregnancy will be difficult. But he said the “only question is how important it is.”
Grunstra maintained, regardless of the evolutionary perspective, “We’re talking about risks. And on a population level, if there’s a higher risk, it means that at some point that’s going to translate into a higher actual rate of complications, things going wrong, and individuals dying. And natural selection absolutely cares about that.”
Are Humans Unique?
Grunstra has in the past pushed back against the claim humans have uniquely difficult births. The new study found that, at least for offspring size, the pattern of larger males held for nearly all mammals.
But Häusler said the sample sizes for many of the groups were too small to draw definitive conclusions from.
Human childbirth is “definitely more difficult than in nonhuman primates and in other animals,” Häusler said. “The risk for dystocia is much higher.”
Wells, Grunstra, and Häusler reported no relevant financial conflicts of interest.
Avery Orrall is a New York City-based medical and investigative journalist working at Retraction Watch. A former healthcare worker, she has also written in JAMA Medical News and Science, among other outlets.
