At the end of the fourth week, the times can be increased up to 60 minutes per session, depending upon the exercise. At this time, strength training can also be added and should be continued twice a week. As a maintenance level, the goal should be 30 to 60 minutes of exercise, three to five times per week.

Stone recommends that "newbies" to the world of exercise consult with a physical trainer to make sure the exercises are being done properly. This information will be particularly useful when you're trying to maintain a workout regimen and avoid injury during pregnancy.

You can maintain your fitness regimen when you're pregnant. Even joggers can continue – with their doctor's approval – through the first trimester.

"The key is to listen to your body's signals," says Stone. "If after the first week your knees are killing you, you're probably doing something wrong, overdoing it or doing the wrong activity."