Women who want to maintain a high exercise level during pregnancy may be frustrated at the outdated ("keep your heart rate below 140") and vague ("stop exercising if you feel tired") information they find.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, Ph.D., 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.
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 pre-pregnancy. The amount of blood a woman has increased 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.
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.
For women who are already in top shape, exercising during pregnancy is about maintaining their fitness, not about making fitness gains, Pivarnik says. "Even if I had tried to take it to a new level, my body wouldn't have let me," Amy Hyland says. Before Hyland got pregnant with her first son four years ago, she was a triathlete who had completed an Ironman, and she exercised six or seven days a week. She was determined to stay active during pregnancy and continued to run, swim, and even ride her bike. "For me, it was about maintaining fitness, not training hard," she says. Hyland even did a duathlon (a run/bike/run event) at five months. "My doctor just told me to stop if I had any pain." She finished just fine—though certainly not in record time. For most pregnant women, the fear of working out too hard is likely a misplaced worry because the pregnant body has a way of both imposing its own limits and adapting.
While there are some things to avoid, such as scuba diving, horseback riding, or any contact sport that could cause blunt-force trauma to the abdomen, there's relatively little that pregnant women can't do. Even the longstanding prohibition against exercising on your back is somewhat of a myth, Dr. Young says. It's true that lying flat on your back late in your pregnancy can cause your growing uterus to push down on the veins whose job it is to deliver blood, leading to decreased blood flow. "Blood can get shunted away from the uterus, and you might feel light-headed," Dr. Young says. But performing exercises on your back for a short period (such as a series of Pilates moves) is not likely to do any harm, and you would feel uncomfortable long before your blood flow was compromised, he explains.
Hyland didn't just exercise to stay in shape; it also helped lift her mood. "I was so happy to be able to get outside and jog for 45 minutes," she says. In fact, exercise keeps your endorphins (your body's natural "happy" chemicals) flowing, Mikeska says. That's a crucial benefit, especially since we now know that there are more mood disturbances during pregnancy than postpartum, due to the massive influx of extra estrogen and progesterone (hormones linked closely to mood), says Melanie Poudevigne, PhD, health and fitness management program coordinator at Clayton State University, in Morrow, Georgia.
Psychologically speaking, pregnancy can also be stressful because it's full of so many unknowns. "That perceived lack of control makes pregnant women very vulnerable to mood disturbances," Poudevigne says. She and her colleagues recently conducted a study where they led sedentary pregnant women with lower-back pain through a moderate-intensity weight-training program. It not only curbed the pain, it also lifted the women's moods, Poudevigne says. They learned how to monitor their discomfort levels, and came to feel that they were more in control of their own bodies.
Pivarnik suggests that pregnant women who exercise monitor signs of potential problems, keeping in mind that if there's something wrong with the pregnancy, it's not caused by exercise. Rather, exercise might provide that extra bit of stress that brings the problem to the forefront. "Don't ignore these symptoms (see below), especially if they keep happening when you exercise," Pivarnik says. Stop exercising and consult your doctor if you have: