Pregnant pre-humans appeared to have stood the same way. And it may save women from even more back pain than they already have, the researchers report in this week's issue of the journal Nature.

The bodies of women do two things when they are pregnant -- they adjust their stance to move the center of gravity to accommodate the growing fetus, and the lower vertebrae have evolved a distinct shape to allow this shifting to take place without damaging the spine, Katherine Whitcome of Harvard University and colleagues found.

"It was one of these things like, 'Oh my god, no one's ever thought of this,' and it looks so obvious," Liza Shapiro of the University of Texas at Austin, who helped supervise the work, said in a telephone interview.

Whitcome and Shapiro followed 19 women through their pregnancy, using digital cameras and motional analysis equipment to map the changes in stance and movement as the months passed.

"What women do when their pregnancy reaches about half of the expected mass ... they shift backwards," Shapiro said.