Pregnant women’s center of gravity

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 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 Thursday’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. 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% larger in women than in men, when body size is taken into account.

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