Normally, medical studies are either good news or bad news. And then there are the ones that make you shrug your shoulders and wonder: why would they study that? Well, a new study will make you do that very thing.

According a report published in Thursday's Nature, scientists believe they know why pregnant women do not topple over. (Have you lower your brow yet?) Despite the fact that woman carry weight in front, researchers think that women's lower backs and hip joints (which differ that men) allow them to adjust their center of gravity.

"That's a big load that's pulling you forward," said Liza Shapiro, an anthropology professor at the University of Texas and the only one of the study's three authors who has actually been pregnant. "You experience discomfort. Maybe it would be a lot worse if (the design changes) were not there."

Harvard anthropology researcher Katherine Whitcomb found two physical differences in male and female backs that until now had gone unnoticed: One lower lumbar vertebra is wedged-shaped in women and more square in men; and a key hip joint is 14 percent larger in women than men when body size is taken into account.

The researchers did engineering tests that show how those slight changes allow women to carry the additional and growing load without toppling over - and typically without disabling back pain.

"When you think about it, women make it look so very damn easy," Whitcomb said. "They are experiencing a pretty impressive challenge. Evolution has tinkered ... to the point where they can deal with the challenge.

"It's absolutely beautiful," she said. "A little bit of tinkering can have a profound effect."

Walking on two feet separates humans from most other animals. And while anthropologists still debate the evolutionary benefit of walking on two feet, there are notable costs, such as pain for pregnant females. Animals on all fours can better handle the extra belly weight.