Research confirms what many women already know. After you give birth, you’ll probably have flat feet that require a bigger shoe size.

“I had heard women reporting changes in their shoe size with pregnancy, but found nothing about that in medical journals or textbooks,” says researcher Neil Segal of the University of Iowa. “We found that pregnancy does indeed lead to permanent changes in the feet.”

The study followed 49 pregnant women and collected static and dynamic arch measurements during the first trimester of pregnancy and again about five months after childbirth. The researchers found that for about 60 to 70 percent of the women in the study, their feet became longer and wider.

The study also suggested that first pregnancies may account for most of the observed changes, while second, third or higher pregnancies may not further alter foot structure.

“We know that women, and especially women who have had children, are disproportionately affected by musculoskeletal disorders,” says Segal. “It is possible that these foot changes that occur during pregnancy may help explain why, in comparison with men, women are at higher risk for pain or arthritis in their feet, knees, hips, and spines.”