Women's spines have evolved over thousands of years, allowing them to carry babies for nine months while providing a great deal of strength and flexibility, researchers say.

Women's lower backs have become more curved to allow them to move more freely while supporting a progressively heavier load, say researchers, whose findings will be published in the Dec. 13 issue of Nature.

"Pregnancy presents an enormous challenge for the female body," Katherine Whitcome, a postdoctoral researcher in the Department of Anthropology in Harvard's Faculty of Arts and Sciences, said in a 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."

After studying 19 pregnant women 20 to 40 years old, researchers found the curve of the spine in the lower back, called the lordosis, can increase by as much as 60 per cent by the end of pregnancy. They noted key differences between men and women in lumbar extension as well.

"In females, the lordosis is subtly different than that of males, because the curvature extends across three vertebrae, while the male lordosis curves across only two vertebrae," says Whitcome. "Loading across three vertebrae allows an expectant mother to increase her lordosis, realigning her centre of gravity above her hips and offsetting the destabilizing weight of the baby." 

Women also possess larger spinal joints that flare out more than in men and are located further down the spine.

Researchers believe that over thousands of years, the evolution of the spine helped women have increased mobility during their pregnancies, increasing their survival.