If you were exercising prior to conceiving, then you're generally safe to continue during pregnancy. Regardless, you should consult your doctor/midife.
That said, walking and swimming are two of the safest exercises you can do during pregnancy. With the okay of a doctor/midwife, you can lift weights, bike, even run if you were before pregnancy. Prenatal yoga and Pilates are fantastic during pregnancy, and can help you stretch without injuring yourself. Pregnancy makes us injury-prone, because relaxin (the hormone that opens up your pelvis) allows ligaments and tendons to stretch beyond their normal capacity, which can damage them.
Avoid any exercises that have you laying on your back and any contact sports.
Listen to your body. If it hurts, STOP. If you get light-headed, STOP. Drink plenty of water, and never exercise alone while pregnant, in case you pass out.
at 1:44 AM on Nov. 12, 2010