Evolution Tips the Balance in Mom's Favor

By Serena Gordon
HealthDay Reporter
Thursday, December 13, 2007; 12:00 AM

Copyright © 2007 ScoutNews, LLC. All rights reserved.

WEDNESDAY, Dec. 12 (HealthDay News) -- Given that a woman's abdominal mass increases by a third during pregnancy, it's a miracle that such a significant shift in gravity doesn't have her tipping over during her final trimester.

Now, a new study describes a surprising set of anatomical reasons why the delicate balancing act works.

According to researchers, the female spine has evolved differently, most likely to accommodate the tremendous demands of pregnancy. Without these evolutionary adaptations, women would probably experience even more back problems than they already do during the gestational period.

"The maternal center of mass shifts forward about three to five centimeters during pregnancy, so that it's no longer beautifully aligned with the hips and feet," said study author Katherine Whitcome, a postdoctoral researcher at Harvard University's department of anthropology who began the research as a graduate student at the University of Texas at Austin.

Whitcome explained that the female body compensates for this shift in several ways. One is to recruit more back muscles. Solely using these muscles, Whitcome said, would quickly cause muscle fatigue and make women more prone to injury. Instead, the female back has evolved to allow three lower vertebrae to form a larger curve to support the growing fetus. In men, only two vertebrae form this curve, called lordosis, she explained.

Females also have a key hip joint that is larger and can flare out further, according to the study, published in the Dec. 13 issue of Nature.

"It's a nifty little package of adaptations. We see it in modern humans, but it's also apparent from the little bit of vertebral anatomy we see in earlier hominids," said Whitcome. "It seems part and parcel of the challenges of bipedalism [walking on two feet rather than four]." These changes would have allowed women in prehistoric times to continue to forage for food, as well as quickly avoid predators, she said.

Whitcome's study included 19 pregnant women between the ages of 20 and 40. The researchers found that when the women were standing, they naturally increased their spine's lordosis, sometimes by as much as 60 percent. Even when the women extended their hips only slightly, their lower backs were extended by as much as 28 degrees.