For millions of years, the human pelvis has evolved under the pressure of conflicting demands—namely, the need to walk efficiently on two legs and the challenge of giving birth to large-brained babies. This evolutionary balancing act is at the heart of the long-debated “obstetrical dilemma,” a hypothesis suggesting that the narrowing of the birth canal to support upright walking has complicated childbirth in humans. First introduced in the 1960s, the idea has remained controversial, in part because of limited data and methodological challenges.
In a new study, researchers tackled this evolutionary puzzle with the help of modern tools: deep learning and large-scale genetic analysis. Using dual-energy x-ray absorptiometry (DXA) scans from over 31,000 participants in the UK Biobank, the team measured seven different pelvic traits—including three that directly relate to the birth canal. They then performed a comprehensive genome-wide association study (GWAS) to uncover the genetic factors influencing pelvic shape, linking these traits to real-world outcomes like childbirth complications, mobility, and pelvic floor health.
One of the key takeaways is that pelvic shape is highly heritable, with genetics explaining between 32% and 48% of the variation in pelvic proportions. The researchers identified 180 genetic loci associated with these traits. Intriguingly, the subpubic angle—a feature central to the width of the birth canal—showed notable genetic differences between men and women, likely reflecting its critical role in female reproduction.
The study also revealed that pelvic asymmetry (differences between the left and right sides) is not inherited but instead tied to handedness. This suggests that developmental or behavioral factors may shape pelvic asymmetry more than genetics.
Linking pelvic dimensions to childbirth outcomes, the findings support the idea that narrower birth canals are associated with increased risks of emergency C-sections and obstructed labor. However, the trade-offs don’t end there. While wider birth canals may reduce childbirth complications, they appear to come at a cost to locomotion—slower walking speeds—and higher risks of hip osteoarthritis. They’re also tied to increased rates of pelvic floor disorders such as genital prolapse and incontinence, which may exert their own evolutionary pressures against expanding the birth canal too much.
Interestingly, the researchers found no genetic connection between pelvic shape and gestational length, which challenges the notion that earlier births evolved to resolve the obstetrical dilemma. Instead, they found that the widths of both infant and adult heads are genetically linked to maternal birth canal dimensions. This supports the idea of coevolution between the size of the human brain and the pelvic structure that must accommodate it during childbirth.
Ultimately, this study brings new clarity to an old evolutionary debate. While bipedal locomotion was long thought to be the primary driver of pelvic form, these results highlight a more complex interplay—where childbirth, pelvic floor health, and brain development have all shaped the unique structure of the human pelvis. The obstetrical dilemma, it turns out, might not be a simple tug-of-war between walking and birthing, but a three-way negotiation that has sculpted human evolution in subtle and surprising ways.