Three days before Nancy Karabaic's baby was born -- when most moms-to-be are finding it tough enough just walking a couple of blocks -- Karabaic took a 3.5-mile run, because it felt good, she says.

"It was really important to me to stay fit and strong throughout my pregnancy," says the 39-year-old personal trainer from Wheaton, Md. "There's so little you can control when you're pregnant that at least you can feel like you're doing your best to stay healthy. I felt it would make my baby healthier, too."

So after talking over safe and reasonable guidelines with her midwife, Karabaic continued to swim, bike, run and lift weights -- albeit at a lesser intensity than she was used to -- throughout the nine months of her pregnancy.

To Karabaic, who delivered a healthy 7-pound, 6-ounce boy last January, the benefits were clear.

"My health stayed really good, I felt better emotionally, and I still felt like I looked good -- I had that healthy glow you get when you're exercising and you're pregnant." She's also convinced her labor, delivery and recovery were smoother because she was in such good shape.

The average pregnant woman won't be taking a three-mile jaunt, however, nor should she. Still, recent studies on pregnant women like Karabaic are shattering a lot of myths about the potential dangers of moderate-to-strenuous exercise during pregnancy.

That's good news for women who have been fairly active before conceiving. It's also reassuring news to those, even novices, who are concerned that exercise might hurt them or their baby.

"Exercise is not a process that needs be eschewed or prevented during pregnancy," says Dr. John Botti, director of maternal-fetal medicine at Penn State Geisinger Health System, who studied the effects of exercise on moderately conditioned pregnant women. "Reasonably performed exercise doesn't appear to cause harm, and may, in fact, have benefits."

The key is finding a reasonable level, and that depends largely on the shape you were in before, the activities your body was comfortable with, and your health during pregnancy. Always check with your doctor or midwife first, but here's food for thought for the novice and the enthusiast, as well as some basic dos and don'ts.

The Novice: Walking and Water Spell R-E-L-I-E-F

"Only 20% to 30% of the population exercises on a regular basis, so the typical pregnant woman hasn't exercised prior to pregnancy," says Bonnie Berk, creator of MOTHERWELL, a pre- and postnatal fitness program offered throughout the United States and abroad.

Still, it's not too late for pregnant women who haven't been consistent exercisers to start now. Although hard data on the value of prenatal exercise isn't as well-documented for unfit women as for fit ones, experts like Berk have seen firsthand the difference that exercise can make, even for couch potatoes.

The Novice: Walking and Water Spell R-E-L-I-E-F continued...

For one thing, by doing exercises that strengthen the muscles supporting the uterus, women stand to experience fewer complications like backaches, ankle swelling and fatigue during pregnancy, and their bodies will be better prepared for the rigors of childbirth, too.