Why Pregnant Women Don't Tip Over

Dec. 12, 2007 -- Pregnant women can thank their aching backs for keeping them upright, a new study shows.

The study, published tomorrow in Nature, explains that women's spines are built differently from men's spines. 

The study shows that the lower part of a woman's spine is built to curve more during pregnancy. That adjustment helps women hold their center of gravity while pregnancy pushes their waistline way beyond their hips.

"Pregnancy presents an enormous challenge for the female body," researcher Katherine Whitcome, PhD, says in a news release.

"The body must change in dramatic ways to accommodate the baby, and these changes affect a woman's stability and posture. It turns out that enhanced curvature and reinforcement of the lower spine are key to maintaining normal activities during pregnancy," says Whitcome, who is a postdoctoral researcher at Harvard University's anthropology department.

(What amazing changes has your body endured during your pregnancy? Share your stories and insights on the Pregnancy: 2nd Trimester board.)

Eons of Pregnancy Back Pain

Whitcome and colleagues studied 19 pregnant women, analyzing how the women stood and walked at various points during pregnancy.

As the pregnancies progressed, the women had more curvature of their lower spine.

This has happened for at least 2 million years, judging by fossils of the earliest hominids that walked on two feet, Whitcome's team notes.

Curving the spine during pregnancy probably never felt good.

"It is reasonable to hypothesize that fatigue and pain in the lower back muscle affected early hominid mothers just as they do modern mothers," write the researchers.

But today's pregnant women are better off in at least two ways, according to the study.

First, a modern pregnant woman isn't likely to be preyed upon by a wild animal when her back pain gets to her. Second, she's got fewer vertebrae than her ancient ancestors, making it easier to carry things, walk, and run, if need be.

"Any mother can attest to the awkwardness of standing and walking while balancing pregnancy weight in front of the body," Liza Shapiro, PhD, says in a news release.

"Yet our research shows their spines have evolved to make pregnancy safer and less painful than it might have been if these adaptations had not occurred," says Shapiro, an associate professor of anthropology the University of Texas at Austin.

Whitcome and Shapiro worked on the study with Daniel Lieberman, PhD, professor of biological anthropology at Harvard University.