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.

However, despite what appear to be nature's best intentions, back pain is still a common complication during pregnancy.

"Women, by and large, make pregnancy look so easy, but there are very few women who have experienced pregnancy who haven't had some back pain," said Whitcome. But, she added, "without these adaptations, I think this back pain would be exceedingly worse. I think these adaptations mediate some of the intensity of back pain."

Dr. Miriam Greene, an obstetrician at New York University Medical Center, agreed that "there's plenty of back pain in pregnancy," but said she wasn't convinced that the differences in male and female spines account for the majority of a woman's pregnancy stability.

"I think the reason why pregnant women don't tip over is that the placenta produces relaxin, a hormone released in pregnancy that spreads your pelvis, making it wider, and giving you a wider stance. That's why you walk like a duck when you're pregnant," she said.

Greene said a pregnant woman's center of gravity starts to shift slowly as she gains weight, with the majority of the shift occurring in the third trimester.

To prevent falls and back strain, Greene recommends wearing flat shoes, ditching heavy bags, taking extra care anytime you lift objects, and perhaps wearing a pregnancy belt. Additionally, she said that pregnancy exercise classes can help maintain the strength in your back.

More information

To learn more about preventing back pain in pregnancy, visit the American College of Obstetricians and Gynecologists.

SOURCES: Katherine Whitcome, Ph.D., postdoctoral researcher and instructor, department of anthropology, Harvard University, Boston; Miriam Greene, M.D., obstetrician/gynecologist, New York University Medical Center, clinical assistant professor, New York University School of Medicine, New York City, and co-author. Frankly Pregnant: A Candid, Week-by-Week Guide to the Unexpected Joys, Raging Hormones and Common Experiences of Pregnancy; Dec. 13, 2007, Nature

Copyright © 2007 ScoutNews, LLC. All rights reserved.