Women's backs evolved for pregnancy

Women's backs evolved for pregnancy

Women’s spines evolved with a more pronounced curve to help them bear the strain of pregnancy, according to research spearheaded at UT Austin.

The natural adaptation eases pressure on a pregnant woman’s back and even limits the damage done when she leans back or hyperextends her spine. The difference between men’s and women’s spines is documented for the first time in the Dec. 13 issue of Nature.

Researchers attribute the difference to an adaptation that first appeared 2 million years ago in an early human ancestor. Because the difference doesn’t appear in chimpanzees, researchers believe that walking upright led to the adaptation.

“Without the adaptation, pregnancy would have placed a heavier burden on back muscles, causing considerable pain and fatigue and possibly limiting foraging capacity and the ability to escape from predators,” said UT anthropologist Liza Shapiro, who conducted the research with graduate student Katherine K. Whitcome, now a postdoctoral fellow at Harvard University.

Harvard anthropologist Daniel Lieberman also contributed to the study, which shows that the key differences between males and females appear in the lower back, or lumbar portion of the spine.

Human spines have a unique forward curve in the lumbar region, but the curve extends across more vertebrae in females. The joints between the vertebrae also are larger in females and angled differently from those of males to better support the extra weight.

“Any mother can attest to the awkwardness of standing and walking while balancing pregnancy weight in front of the body,” Shapiro said. “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.”