Women who want to maintain a high exercise level during pregnancy may be frustrated at the outdated ("keep your heart rate below 140") and vague ("stop exercising if you feel tired") information they find. "Physicians aren't trained to counsel pregnant women about exercise," says James Pivarnik, PhD, vice president of the American College of Sports Medicine. "It's a rare bird who keeps up with the exercise and pregnancy literature." Doctors may have you believe that we know little about how exercise affects pregnant women. But in reality, we know quite a bit -- and it's good news for runners, cyclists, and gym rats who are moms-to-be. "If a woman is having a normal pregnancy, she can continue to exercise, and the upper limit of the level can be reasonably close to what she was doing before pregnancy," Pivarnik says.
Will Exercise Harm My Baby?
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.
How Will I Feel While Exercising During Pregnancy?
Pregnant women often notice that they feel out of breath more quickly than they used to. You may assume this is a sign that you're out of shape. In fact, during pregnancy you're breathing 20 to 25 percent more air because you need to get rid of the carbon dioxide levels in your own blood -- and in your baby's. (Babies in utero aren't breathing on their own, but they're still producing carbon dioxide, which transfers to the mother's blood. She needs to breathe more so she can get rid of it.) "So breathing more doesn't mean you're any less fit," explains Dennis Jensen, PhD, lead researcher on a Queen's University study of exercise and respiratory discomfort during pregnancy. It simply means that your body is adapting exactly as it should.
Jensen's research found that when pregnant women exercised to fatigue on a stationary bike at 20, 28, and 36 weeks, their maximal aerobic capacity (how hard they could work) was well preserved, even though they were breathing more.
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