For most women, exercise during pregnancy is safe. In fact, several studies suggest that women who exercise during pregnancy will have a healthier pregnancy. We don't know, however, if the exercise or other health habits of the exercising women are responsible for the improvement.

Most types of exercise -- in moderation -- are OK for pregnant women. By moderation I mean that the woman works to maintain her fitness rather than pushing herself to a new level of performance.

As the pregnancy advances, it will be harder to keep up the pace, endurance and routine of any exercise routine the woman had before she became pregnant. A ten-minute mile, the yoga pose called "downward facing dog," or touching your toes may be easy in the first trimester, but for many, these will be nearly impossible in the third trimester.

Women who exercise during pregnancy should drink plenty of water. Also, slow down and reduce your level of exertion if you feel dizzy or exhausted. Avoid exercises and activities in which there is a risk of falling and hitting your abdomen -- or having others hit you. Avoid activities that require good balance. Your center of gravity changes during pregnancy, increasing the risk of a fall.

In the second half of pregnancy, avoid activities that have you flat on your back for more than 20 to 30 minutes at a time. This position can reduce blood flow to the uterus.

Finally, swimming is great exercise during pregnancy, but stay away from scuba diving. "The bends" (bubbles of gas in the blood stream) are dangerous to the fetus.

The American College of Obstetricians and Gynecologists says that "recreational and competitive athletes can remain active during pregnancy." However, in certain cases, doctors may recommend limited activity. These conditions may include:

• bleeding during pregnancy, especially heavy recurrent bleeding late in pregnancy
• a placenta that covers the cervix (placenta previa) in the third trimester
• a history of prior preterm birth

If you did not exercise regularly before pregnancy, have a medical condition (such as heart disease) that may limit activity, or have had or have pregnancy complications, check with your doctor or midwife before planning any exercise during your pregnancy.