Washington ? Scientists think they have figured out why pregnant women don’t lose their balance and topple over despite ever-growing weight up front.

Evolution provided them with slight differences from men in their lower backs and hip joints, allowing them to adjust their center of gravity, new research shows.

This elegant engineering is seen only in female humans and our immediate ancestors who walked on two feet, but not in chimps and apes, according to a study published in today’s journal Nature.

“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 been pregnant. “You experience discomfort. Maybe it would be a lot worse if (the design changes) were not there.”

Harvard anthropology researcher Katherine Whitcome 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 changes allow women to carry the load without toppling – and typically without disabling back pain.

“It’s absolutely beautiful. A little bit of tinkering can have a profound effect,” Whitcome said.

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.

But what about men with stomachs the size of babies or bigger? What keeps them from toppling over?

Their back muscles are used to compensate, but that probably means more back pain, theorized Shapiro.