When not to exercise: you have a history of miscarriage, you experience any bleeding, you have pregnancy induced high blood pressure, you have aasthma. 

First and Second Trimester

While your body still looks like it did before, there are many small changes going on inside during your first trimester of pregnancy. Many women don't feel much different, at least early on in the pregnancy, and there's little risk to the baby; so, this is a great time to continue or regulate your exercise program and to think about the benefits of exercise. Exercise in pregnancy has a multitude of benefits-from improving your sleep to making labor and delivery easier, to providing endorphins and making you feel better, and to helping you get your figure back more quickly after delivery. 

Aerobics, yoga and relaxation techniques, Kegel exercises, and weight training can be done safely during the first trimester of pregnancy.  Activities that put you at a higher risk of injury-such as downhill skiing, horseback riding, mountain climbing, and contact sports like soccer and basketballshould be avoided during pregnancy. 

As your pregnancy progresses, the extra weight and its unwieldy distribution place stress on your joints and muscles, especially in the lower back and pelvis. You might also have problems with circulation, causing leg cramps and dizziness. Adapt your exercise regimen accordingly. If you are still engaging in rigorous workouts, such as cycling or step exercises, this would be a good time to shift to less strenuous activities — and those that don't require careful balance. As your fetus has grown, your center of gravity has shifted. You also may have less oxygen available, so reduce the pace of your routines, or stop altogether if you become breathless. 

After the first trimester, avoid exercises that require lying flat on your back. The weight of your expanding uterus can compress major blood vessels and restrict circulation. Do your abdominal exercises in a standing position, and other floor exercises lying on your side. 

Overheating during exercise can be dangerous. Keep your body temperature at a moderate level. An increase of more than one degree of body heat can be dangerous. If you're not sure, wear a monitor.

Third Trimester

Women who are in good general health and whose pregnancies have progressed normally for the first two trimesters are encouraged to exercise within comfortable limits. Walking is an excellent way to keep up fitness and dancing can be a great deal of fun. Putting on some upbeat music while doing housework is one of the best ways to elevate the heart rate and be productive at the same time! Be sure to ask your practitioner about exercise every visit to ensure you're both on the same page. This will also help both of you stay informed about your abilities and exercises.  There are no specific recommendations to stop exercising prior to labor. Many women go through pregnancy and exercise until the day of labor. Other moms might feel tired and worn-out, decreasing their workouts in intensity and frequency as they go. Either way is fine as long as it is what is right for you and your baby.

From about half way through the pregnancy, you should avoid any exercises that require you to lie on your back, to avoid placing any undue stress on your spine. Always drink lots of water to keep dehydration at bay and don't do any strenuous activities during hot, humid weather. You should always avoid overheating your body when pregnant. Loose, comfortable clothing is a must, as is a well-fitting, supportive bra that will keep your breasts from moving around too much.

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