Once upon a time, pregnant moms were told to keep off their feet and avoid exercise, but today's experts say quite the contrary. "Exercise is great. I recommend it," 'says Sara Duhn, a nurse practitioner in the San Francisco Bay Area with more than 20 years experience. Walking is a good mom-to-be workout, but marathon runners and other fitness fans need not downsize to strolling. Here are a few guidelines to keep in mind:

You sweat more during pregnancy, so drink plenty of fluids.

After your fifth month, avoid exercising on your back, as this decreases blood flow to you and your baby.

If you find yourself out of breath, decrease the intensity.

Avoid any exercise that could cause you to fall: horseback riding, downhill skiing, rock climbing, and so on.

Work longer, not harder. You want to build up your stamina for labor Duhn says. Walking for 40 minutes at a moderate pace is better than 10 minutes of fast walking.

If you haven't been exercising, begin slowly, with 10-minute walks 5 times per week and gradually increase the duration of your walks. If you need motivation, go with your partner-it's a nice way to connect. Swimming and prenatal yoga are great options, too.