However, even if you are fit, pregnancy is not a good time to strive for exercise milestones. It is not the time for you to try to lose weight, or to increase your exercise intensity levels. Your doctor may even set upper limits for your heart rate during exercise. (A heart-rate monitor is an ideal tool to provide you with the best measure of your exercise intensity.)
To find the best-and safest-exercise for you, you need to speak to your obstetrician. It's particularly important to do so during your third trimester because recent research suggests that vigourous exercise of the lower extremities can cause problems for your fetus during this period.
A doctor will often encourage a pregnant woman to strengthen and tone her torso, back, pelvis, thighs, and stomach because these help with the delivery process. (See your health care provider for a list of specific activities offered by the American College of Obstetrics and Gynecology.) Whatever exercises you do, exercise to the point of mild fatigue and never to the point of exhaustion.
Contact sports or extreme sports (for example, skiing, scuba diving, and sky diving) should be avoided because they have the potential to do harm to you or your baby. And after your first trimester, activities that involve lying flat on your back for long periods of time should be avoided. That's because the weight of your developing baby may press down on and compress blood vessels that play an important role in blood circulation, both for you and your baby. As fatigue tends to be most common in the first and third trimesters, don't be discouraged if you find yourself less able to exercise during these periods.