On average, a woman's foot can grow by a half size or more during pregnancy. "The same hormones that are released to relax the pelvic area also loosen the ligaments in the foot," says Bret Ribotsky, president of the American College of Foot and Ankle Orthopedics and Medicine. "In addition, the increased body weight of pregnancy lowers the arches, further adding to the foot's length and width."

Because the expansion is gradual, you might not realize the change. Shoe-size vanity (thinking that smaller is better) may also play a role. But ultimately, not buying larger shoes becomes self-defeating. "Too-tight shoes weaken the muscles in the ball of the foot and the ligaments that hold the toes straight," says Ronald Smith, M.D., associate professor of orthopedic surgery at Harbor-UCLA Medical Center. "And that can set the stage for foot problems."

These can include corns (caused by pressure from the shoes), ingrown nails, and bunions (protrusions at the base of the big toe that grow larger when rubbed by a shoe). The higher the shoe's heel, the likelier it is that you'll have foot problems.

To find the perfect fit: Make sure the shoe doesn't rub against you uncomfortably and is roomy enough to provide a finger's breadth between its tip and your big toe. To test the fit, trace a shoe onto a piece of paper, then trace your foot on that same paper. Compare the two. If your foot is as big as or bigger than the shoe, you need a larger size.

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