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 actually 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 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,” Whitcome 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.”

When the researchers looked back at fossil records of human ancestors, including the oldest spines that go back 2 million years to our predecessor, Australopithecus, they found a male without the lower-back changes and a female with them.