Evolution Tips the Balance in Mom's Favor

Evolution Tips the Balance in Mom's Favor
WEDNESDAY, Dec. 12 (HealthDay News) -- Given that a woman'sabdominal mass increases by a third during pregnancy, it's a miracle thatsuch a significant shift in gravity doesn't have her tipping over duringher final trimester.
Now, a new study describes a surprising set of anatomical reasons whythe delicate balancing act works.
According to researchers, the female spine has evolved differently,most likely to accommodate the tremendous demands of pregnancy. Withoutthese evolutionary adaptations, women would probably experience even moreback problems than they already do during the gestational period.
"The maternal center of mass shifts forward about three to fivecentimeters during pregnancy, so that it's no longer beautifully alignedwith the hips and feet," said study author Katherine Whitcome, apostdoctoral researcher at Harvard University's department of anthropologywho began the research as a graduate student at the University of Texas atAustin.
Whitcome explained that the female body compensates for this shift inseveral ways. One is to recruit more back muscles. Solely using thesemuscles, Whitcome said, would quickly cause muscle fatigue and make womenmore prone to injury. Instead, the female back has evolved to allow threelower vertebrae to form a larger curve to support the growing fetus. Inmen, only two vertebrae form this curve, called lordosis, sheexplained.
Females also have a key hip joint that is larger and can flare outfurther, according to the study, published in the Dec. 13 issue ofNature.
"It's a nifty little package of adaptations. We see it in modernhumans, but it's also apparent from the little bit of vertebral anatomy wesee in earlier hominids," said Whitcome. "It seems part and parcel of thechallenges of bipedalism [walking on two feet rather than four]." Thesechanges would have allowed women in prehistoric times to continue toforage for food, as well as quickly avoid predators, she said.
Whitcome's study included 19 pregnant women between the ages of 20 and40. The researchers found that when the women were standing, theynaturally increased their spine's lordosis, sometimes by as much as 60percent. Even when the women extended their hips only slightly, theirlower backs were extended by as much as 28 degrees.
However, despite what appear to be nature's best intentions, back painis still a common complication during pregnancy.
"Women, by and large, make pregnancy look so easy, but there are veryfew women who have experienced pregnancy who haven't had some back pain,"said Whitcome. But, she added, "without these adaptations, I think thisback pain would be exceedingly worse. I think these adaptations mediatesome of the intensity of back pain."
Dr. Miriam Greene, an obstetrician at New York University MedicalCenter, agreed that "there's plenty of back pain in pregnancy," but saidshe wasn't convinced that the differences in male and female spinesaccount for the majority of a woman's pregnancy stability.
"I think the reason why pregnant women don't tip over is that theplacenta produces relaxin, a hormone released in pregnancy that spreadsyour pelvis, making it wider, and giving you a wider stance. That's whyyou walk like a duck when you're pregnant," she said.
Greene said a pregnant woman's center of gravity starts to shift slowlyas she gains weight, with the majority of the shift occurring in the thirdtrimester.
To prevent falls and back strain, Greene recommends wearing flat shoes,ditching heavy bags, taking extra care anytime you lift objects, andperhaps wearing a pregnancy belt. Additionally, she said that pregnancyexercise classes can help maintain the strength in your back.