The first thing that most newly pregnant exercisers worry about is miscarriage -- thanks to age-old myths that have women believe that a bout of strenuous exercise can harm the baby. "There is no real evidence that exercise is linked to miscarriage," says Bruce K. Young, MD, coauthor of Miscarriage, Medicine & Miracles (Bantam) and professor of obstetrics and gynecology at New York University School of Medicine. Heavy exercise isn't going to hurt your baby, but it will tire you more quickly than it did prepregnancy. The amount of blood a woman has increases during pregnancy by about 50 percent, and her heart needs to work harder to push all that blood around -- including circulating it through the placenta, an extra organ. "That means the stress on your heart will be 50 percent greater for the same exercise that you were doing before pregnancy," Dr. Young says. So you can work just as hard doing less than you did before you were pregnant.
Pregnancy isn't the time to push yourself to the max, but it's also okay -- and good for you -- to get your heart rate up with cardiovascular exercise. Although a target heart rate of 140 is a number that's often cited, there's no precise number to shoot for. When prenatal trainer Erinn Mikeska, owner of Delivering Fitness, in Dallas, works with pregnant women, she has them monitor their rate of perceived exertion (RPE) -- how hard they feel they're working, on a scale from 1 to 10. "You probably want to stay around 5 or 6," she says. In the first trimester, when you're not any bigger and don't yet have balance issues, you may be able to exert yourself more if you're not too tired.