We can all thank our mothers for our big brains. If it weren’t for the relatively wide female pelvis, which develops during puberty, we would never have been able to get our ever-growing noggins through the birth-canal bottleneck. Now, by charting the course of pelvic development across the human life span, researchers have found that a woman’s pelvis narrows as she gets older, marking the end of her childbearing years. It’s “like a window that opens for a certain time, and afterward closes,” says paleoanthropologist and study author Marcia Ponce de León of the University of Zurich in Switzerland.

The pelvis has long fascinated researchers because it’s “a literal gateway to evolution,” says evolutionary anthropologist Holly Dunsworth of the University of Rhode Island, Kingston, who was not involved in the study. “You either pass through that and your lineage goes on, or you’re blocked and you go extinct.”

In the past, scientists studying how age alters the human pelvis have analyzed 2D depictions of the pelvic bones. Now, they are watching how the 3D architecture of the entire pelvis develops over a human life span, using sophisticated imaging technology.

“Today we are in the great situation that we actually can track development in three dimensions because we have these full body biomedical imaging data sets,” says paleoanthropologist Christoph Zollikofer of the University of Zurich, an author of the study. “Before, it was simply impossible.”

Zollikofer, Ponce de León, and their co-authors took CT scans of the pelvises of 275 individuals—151 males and 124 females—who ranged in age from developing infants to 95 years old. The scans allowed the researchers to create 3D animations of each pelvis. Then, they identified anatomical landmarks on the bones, which they used to analyze how the shape of the pelvis differed between males and females and between people of different ages.

The 3D models showed that by the end of puberty, the average female pelvis was about 25% wider than the average male pelvis, as expected. The analysis also uncovered a surprising pattern: The pelvises of females aged 70 and above were about 8% narrower than those of middle-aged females, suggesting that the female pelvis constricts in older adults, the scientists report online today in the Proceedings of the National Academy of Sciences. Despite that narrowing, the pelvises of older females were still wider than male pelvises of the same age. The period when the female pelvis is widest—between puberty and age 40—coincides with the time of peak fertility.

But why does the female pelvis narrow at all? The structure provides support for internal organs, like the uterus, that rest on top of it. A narrower opening in the pelvis means those organs are sitting on a more stable surface: bone instead of muscle and other soft tissue. The researchers suggest that the changing shape of the female pelvis may provide a woman’s body with a wider birth canal when it’s needed—during the period when she’s most likely to have a baby—and more structural support at other times. The authors point to estrogen levels, which rise during puberty and decline later in life, as the likely cause of the widening and subsequent narrowing in the female pelvis, in particular because estrogen is known to impact bone growth and development.

But that hypothesis still needs to be tested, says paleoanthropologist Caroline VanSickle at the University of Wisconsin, Madison. Furthermore, because the researchers were studying modern humans, the results may reflect recent changes rather than evolutionary ones, Dunsworth says. “Putting it in evolutionary context is going to require more work,” she adds. Still, she says, the researchers “really enhance our understanding by showing us what these changes look like over the entire life course.”