Exercise during pregnancy should always be discussed with your midwife or ob/gyn. There may be special considerations that you need to consider. This is what I tell my patients.
If you were not exercising on a regular basis prior to the pregnancy you should probably limit yourself to 15 minutes of sustained exercise and be supervised starting out. You can monitor your heart rate or do the talk test to assure you are at an appropriate intensity.
If you were doing a structured exercise program at least 3x a week or an athlete in training you may be allowed to continue exercising with appropriate exercises for pregnancy for 30-45 minutes 3x a week.
Exercise during pregnancy should not cause pain or breathlessness or be done at such an intensity that you are working up an extreme sweat or overheating yourself and the baby. Remember to keep your target heart rate at 50 percent and to stay well hydrated.
Certain conditions in pregnancy prohibit exercise. These include pre-eclampsia, pregnancy induced hypertension, premature rupture of membranes and preterm labor, incompetent cervix, persistent bleeding in the second trimester, intrauterine growth retardation, multiple gestations at risk for preterm labor, and patient with restricted lung and cardiovascular disease.
It may not be a good idea if you have asthma, anemia, a compulsive disorder to exercise or if trying to find time to exercise is causing extreme stress. Write down and discuss your questions about pregnancy and exercise at your next prenatal visit.