Before starting any exercise program, talk to your midwife or physician to make sure it's safe for you to begin. Common reasons to avoid exercise in pregnancy include vaginal bleeding or if you have a history of pre-term labor. In fact, swimming is an ideal exercise for pregnant women, along with low-impact aerobics, walking and dancing. Weight training is also fine, in moderation. It is critically important to drink enough water while exercising and to avoid exercising outdoors when the weather is hot and humid.
Dehydration is dangerous for the mother and fetus.
The baby's temperature is higher than yours to begin with and increases slightly when you exercise. That's why drinking water is so important. If you are a trained athlete such as a runner or a cross-country skier, you will probably be able to pursue your sport throughout the pregnancy. Expect to slow down in the last half of the third trimester, and remember that you may have a hard time catching your breath. Your body will change in a number of ways to accommodate the growing fetus.