Neil Segal, a (rehab) doc at the University of Iowa, heard that some local women had to don larger shoes after pregnancy. So he and colleagues measured the feet of 49 women during their first trimester and again five months after childbirth. Indeed, (nearly) half the women went up a shoe size after childbirth, to accommodate their longer, flatter feet. And multiple pregnancies may add to that effect, Segal says.

Pregnant women produce a hormone called "relaxin," which makes the pelvis more flexible. That's good for childbirth. But the (hormone) can affect joints and ligaments elsewhere, like in the foot. Adding pregnancy weight to that flexibility can lead to (permanent) flattening of the foot's arch—associated with knee and hip pain in older women. Segal's now investigating whether custom shoe inserts can protect women against that risk, and help get motherhood off on the right foot.