Some primates undergo even more extreme childbirth than humans

In the 1990s, a veterinary student took X-rays of squirrel monkeys as they gave birth. The images revealed something startling: the baby’s head was double the size of the mother’s pelvic opening. To get the baby out, the three bones of the pelvis completely dislocated, separating from one another during birth. This does not happen in humans. And it means that, on paper, human childbirth is not the most extreme in the primate world—far from it.

That finding is the centerpiece of a conversation between Science Friday host Flora Lichtman and Dr. Leah Betsy, an anthropologist at University College London. Betsy’s research reexamines the long-standing idea that human childbirth is uniquely difficult because of a conflict between walking upright and having big-brained babies. By comparing humans to a wider range of primates, she argues that the picture is more complicated—and that other species face even steeper mechanical challenges.

The obstetrical dilemma, revisited

For decades, the standard explanation for why human childbirth is hard has been the “obstetrical dilemma.” The idea, first formalized in the 1960s, is that bipedalism reshaped the human pelvis into a narrower, more compact passage, while the evolution of large brains produced babies with big heads. The result is a tight squeeze—a geometric conflict that makes labor long, painful, and dangerous.