Why boys may be harder to deliver: The "costly son" hypothesis explained

It's long been known that male babies tend to be slightly larger at birth — but new research suggests that size difference may have real consequences in the delivery room. A study published in The Anatomical Record analyzed data from 140 mammal species and found that in species where males are larger as fetuses, the risk of mechanical birth complications — including cesarean sections, cephalopelvic disproportion, and shoulder dystocia — is significantly elevated. Researchers from University College London and the University of Vienna are calling this the "costly son" hypothesis.

The findings challenge the idea that difficult human childbirth is purely a result of our big brains and upright posture (the classic "obstetric dilemma"). Instead, fetal sex may be a broader evolutionary factor across mammals — though experts note that the magnitude of its contribution remains debated.

By the Numbers:

• In two-thirds of species studied, male offspring were larger at birth
• In 78% of species with larger adult males, male newborns were at least 3% heavier than females
• Mothers in Nepal giving birth to boys had an 18% higher chance of experiencing obstructed labor (prior research)
• One competing 2026 study found fetal sex accounted for less than 1% of overall cesarean risk

Why it matters: Understanding the evolutionary and biological roots of birth complications tied to fetal sex could help clinicians better anticipate and prepare for difficult deliveries — potentially saving lives, especially in settings where medical intervention isn't readily available.