There certainly are situations where in high risk pregnancies, activity needs to be limited: cervical incopetence, preterm labor, fetal growth and fluid problems, and preeclampsia just to name a few.

However, for the most part, physical activity has great benefits during pregnancy and may:

Decrease c/s rate
Decrease obesity
Decrease the risk of the development of preeclampsia
Decrease the risk of gestational diabetes

Guidelines for excercise are about 150 mins of activity over the week, preferably spread out in 30 minute intervals over 5 days. The best excercises are non-contact excercises to avoid trauma: cycling, swimming, walking/jogging. It is essential to keep track of your pulse as a heart rate of greater than 140 beats per minute decreased uterine blood flow. So keep these guidelines in mind and utilize excercise as another means of helping you get a healthy baby.