Scientists found that childbirth can be harder than humans thought, as many primates have babies much bigger than birth canal; some species even dislocate pelvic bones to get through it

The story Americans have been told for decades goes something like this: walking upright reshaped the human pelvis, our brains got bigger, and our species ended up with the most difficult, riskiest birthing process in the animal kingdom.

A new study is challenging that notion. According to a 2026 study, ‘Comparative primate analysis shows that humans are not unique in having a tight cephalopelvic fit at birth,’ published in Nature Ecology & Evolution by Nicole Torres-Tamayo and colleagues at University College London, difficulty during childbirth is far from a uniquely human problem. Several other primate species face an equally tight, or tighter, squeeze between an infant’s head and the mother’s birth canal.

Where the “humans have it worst” idea came from
The long-standing assumption comes from a landmark study in the 1940s by anthropologist Adolph Schultz, who looked at birth canal sizes among primate species and found most could give birth without much trouble.


The new UCL-led study discovered a major flaw in Schultz’s technique: he used measurement landmarks created specifically for the human pelvis for every other primate species, despite their very different pelvic anatomy. When projected onto non-human primates, those same landmarks tend to form an angled plane above the real birth canal, making the canal seem more spacious than it is.

A much tighter fit than anyone expected
To correct this, the research team built new 3D measurements of the birth canal and compared them to the size of newborn skulls in 29 primate species.

The results of the study indicate that several small-bodied primates experience extreme constriction during birth. According to Nicole Torres-Tamayo and colleagues, tamarins and squirrel monkeys show the most extreme mismatch, with fetal heads nearly twice the size of the available pelvic space, while bushbabies also rank among the species with the most disproportionate cephalopelvic fit.

Co-corresponding author Dr. Lia Betti said the team did not expect to find a mismatch in so many species, and suggested that difficult birth may be the ancestral condition for primates in general, rather than something unique to human evolution.

How other primates cope without C-sections
Without modern medicine, some species have come up with their own workarounds. The pelvic bones of female rhesus macaques fuse later than those of males, during their reproductive years, while the pelvic bones of bushbabies never fuse at all, allowing the pelvis to expand during labor to accommodate the newborn’s head, Betti told in the release for the study.

This aligns with a related 2025 paper on primate pelvic bones. In the study, ‘Variation in Pubic Symphysis Fusion Across Primates: Implications for Obstetric Adaptation,’ published in the American Journal of Biological Anthropology, Torres-Tamayo and colleagues write that delayed or absent fusion of the pubic bones is more common in female primates than males, supporting the idea that flexible pelvic joints may function as a birth adaptation in some species, although the link isn't consistent across every species studied.

We, humans, don’t have this choice. If the pelvis were permanently mobile, bipedal walking would be far less stable, so this sort of flexibility seems to have been ruled out by the bipedal lifestyle of our species.

So is the human pelvis still special?
Yes, kind of. Birth complications are much rarer in great apes overall, the new study says, likely because their larger bodies reduce the mismatch seen in smaller, tree-dwelling primates. This makes humans a little strange, as the only large-bodied ape still subject to a tight cephalopelvic fit.

But not all researchers agree about how special that makes us. A related 2024 study in Nature Ecology & Evolution led by Nicole Webb used three-dimensional simulations to suggest that chimpanzees may also have a constricted midpelvis like humans, pointing to a gradual increase in birth difficulty across ape evolution rather than a sudden, uniquely human change.

Why this matters outside biology class
This is not merely an academic argument. The “obstetrical dilemma” theory has long informed how scientists and the public think about pregnancy risk, and why human infants are born so much more helpless than those of other mammals. If birth difficulty isn’t uniquely human, scientists may have to go even further back, perhaps to the earliest primates more than 50 million years ago, to find its evolutionary roots.

The lesson here is less about anxiety and more about perspective. Human childbirth is genuinely hard, but evolution might have dealt a tough hand to plenty of other species long before humans ever stood upright.