Genetic Match Between Mother and Child Reduces “Obstetrical Dilemma”

In 1960, anthropologist Sherwood Washburn described the “obstetrical dilemma.” As humans evolved new ways of moving and higher intelligence, the female pelvis faced two oppositely opposed pressures—a narrower pelvis is better for efficient upright walking, but a wider one can more easily accommodate large-brained babies. He proposed that evolution had found a clever solution to allow the best of both worlds: human babies are born earlier so they can fit through a narrower birth canal. Although this proposed solution is still firmly lodged in the popular imagination, it has been hotly debated by scientists ever since.

Now, writing in the journal Science, a team from The University of Texas at Austin, Columbia University and Western Washington University has advanced a better explanation. They found that certain genes in the mother determine both the mother’s birth canal size and the baby’s head size at birth, keeping their ratio in proportion with one another. This would make it less likely that a mother with a small birth canal would give birth to a baby with a large head.

“We show genetic correlation between birth canal and head widths,” explained Liaoyi Xu, first author of the study and a graduate student in cell and molecular biology at UT Austin. Xu called this “evidence of co-evolution between human pelvis and brain.”

The team was led by Vagheesh Narasimhan, an assistant professor of integrative biology and statistics and data science at UT Austin.

Using large medical datasets on thousands of mothers and their babies, and applying artificial intelligence tools, the team identified several specific genetically driven traits related to the pelvis and various health and size considerations. Although a physical correlation between a mother’s birth canal size and her baby’s head size had been speculated before, no scientific study had previously confirmed it, much less the genetic factors underlying it.

“The main challenge in previous studies has been the issue of small sample sizes, which has led to inconsistent results,” Xu said. “In our study, we addressed this limitation by using deep learning techniques to extract seven image-derived phenotypes from over 30,000 dual-energy X-ray images. This allowed us to perform the largest and most comprehensive genomic study of pelvic proportions to date. By linking these findings to outcomes related to locomotion, pelvic floor function, and childbirth, we effectively increased the sample size for research on this critical evolutionary topic by more than two orders of magnitude.”

Other researchers have shown Washburn’s proposed solution to the obstetrical dilemma—a shorter gestation—was probably wrong. Compared to other primates, humans actually aren’t born early. 

The UT team also investigated whether another part of Washburn’s proposed dilemma is true, specifically whether walking or running puts evolutionary pressure towards decreasing pelvis size. In analyzing the data, they found mixed results: a wider pelvis does increase the lifetime risk of joint disorders and reduces walking speed, but it also reduces the risks of back pain. 

The team also revealed other clues about the relationship between health outcomes and genetic drivers of pelvic size. 

“Larger birth canals were linked to slower walking pace and reduced back pain, but increased hip osteoarthritis risk,” said Xu, “while narrower birth canals were associated with reduced pelvic floor disorder risk but increased obstructed labor risk.”

A previous paper Narasimhan published in Science in 2023 was celebrated this week at The University of Texas at Austin’s 2025 Celebration of Research, for the campus-wide Research Paper Excellence Award.

Funding for this research came from UT Austin’s Office of the Vice President for Research, Scholarship and Creative Endeavors (Good Systems initiative and the Texas Advanced Computing Center) and from the Paul G. Allen Family Foundation.