When I was a teenager, my mother would look at me with horror every time I laced up my kicks to go running. "You shouldn't do that," she'd say ominously. "You can shake your ovaries loose."

Until recently, even the medical community was ambivalent about prenatal exercise, citing concerns that it could cause miscarriage, poor fetal growth, or injury to Mom. But for normal, low-risk pregnancies, these apprehensions are off base. New research shows that regular exercise will improve your health, stabilize weight gain without affecting the baby's development, help prevent gestational diabetes, allow you to bounce back better post-delivery, and improve your psyche.

Still, it's hard to know exactly what you can and can't do exercise-wise. Here, we outline some common myths and systematically debunk them. Provided your doctor gives you the green light, you can hit the ground walking—with one major caveat: If you experience any dizziness, shortness of breath, or symptoms indicating preterm labor, cease all activity and contact your physician. These are signs that, for you, taking it easy may be the best prenatal fitness regime.

myth one

This notion is rooted in the theory that hoisting dumbbells requires enough exertion to induce premature labor or miscarriage. But according to Michelle F. Mottola, Ph.D., director of the R. Samuel McLaughlin Foundation Exercise and Pregnancy Laboratory at the University of Western Ontario, there have been no reported cases of either one of these scenarios. The real issue is that, because your body releases relaxin (a hormone that softens the tissues around the birth canal, ligaments, and joints) during pregnancy, there may be a risk of dropping a weight and causing injury. But if you shift to lighter weights throughout pregnancy, as Mottola recommends, as well as work slowly and use good form, that's unlikely to happen.

myth two

This outmoded restriction grew out of the idea that elevating the heart rate raises body temperature to a level that can affect the fetus. "It was an old guideline from the American College of Obstetrics and Gynecology [ACOG]," notes Nadya Swedan, M.D., a New York City physiatrist. Because women's fitness levels vary so widely, the ACOG no longer recommends maintaining any specific heart rate. Still, it's best to be cautious. One way is to monitor exertion with the "talk test": See if you can carry on a conversation while exercising. Working out in a cool room in loose clothing is also helpful.

myth three

On the contrary, it may help your baby grow. In a study by the McLaughlin Foundation, participants who exercised two days a week or fewer had lighter babies than those who exercised three or four times a week. But there's a point of diminishing returns; the study also showed that the offspring of women who worked out five or more days a week had weights comparable to those of the couch potatoes' children.

myth four

Actually, there's no better time. "The old rules discouraged women from beginning a fitness routine or trying out a new form of exercise, for fear they would injure themselves," says Swedan. "But it's recommended now that all women exercise while pregnant, as long as they're healthy." Raul Artal, M.D., a member of the ACOG and chairman of the department of obstetrics and gynecology at St. Louis University, concurs: "Pregnancy isn't an illness, something that requires you to live in a state of confinement. It's a time when you should be moving and eating healthfully."

myth five

"There's really no reason why a healthy woman can't continue her usual fitness routine while she's pregnant," says Swedan. "The important thing is to listen to your body. Don't try to work through any pain." And ratcheting your intensity up is definitely not advised. "This isn't the time to lift heavier weights or beat your running time," says Beverly Hills–based celebrity trainer Gunnar Peterson. "Pushing yourself harder while your body is already under added stress can lead you to injure yourself."