Here’s why: A pregnant body produces a hormone called relaxin – which, true to its name, relaxes certain things… Like your pelvic ligaments and joints. It also relaxes ligaments in your feet – so your bones separate a little bit. Add to that the fact that you weigh more when you’re pregnant, and it causes your arches to fall. On average, a woman’s feet will grow by about a half size after having a baby. And that new size is permanent.