There are a multitude of good reasons to stick with (or even start) an exercise routine while you're pregnant. Working out will help you sleep better, boost energy, decrease minor aches and pains, and improve circulation. And since blood volume increases when you're expecting, keeping it flowing will also help curb swelling, leg cramps, varicose veins, constipation and hemorrhoids. Not bad for 30 minutes of exercise three to five times a week.

But while it's important that you stay active, pregnancy isn't a time to be training for the Iron Woman. The American College of Obstetricians and Gynecologists (ACOG) recommends starting an exercise regime, or continuing with your regular workouts, as long as you take a few precautions. Because of pregnancy body changes -- joints that stretch more easily, faltering balance, and increased heart rate -- you're more prone to pulling muscles, falling or tripping, and getting overheated. Follow these tips for a safe and effective pregnancy workout:

Stretch gently. Ligaments are looser during pregnancy, so intense stretching could cause injury.

Don't exercise to lose weight. During a normal pregnancy, women should gain 25 to 35 pounds; with twins or multiples, 35-45 pounds.

Drink plenty of water before, during and after your workout Not only does it keep you hydrated, it helps to regulate body temperature. A good rule of thumb is six ounces of water every 20 minutes.

Stay away from full sit-ups and double leg lifts (raising and lowering both legs at once), or any exercises that require you to lie on your back.

Go for the low-intensity workout. Jumping or jarring movements can affect your balance, and high-impact running or aerobics can overheat your body and cause strain. Walking, swimming, moderate strength training, stationary bikes and elliptical trainers, and pre-natal yoga and aerobics classes are all great options. Intense exercise can direct too much oxygen and blood flow away from the uterus, so keep it at a moderate level. Signs that you're at the right intensity: you can speak normally and breathing is even and moderate. Lenita Anthony, an exercise physiologist and author of Pre- and Post-natal Fitness tells her clients to use the two-hour rule: "If you're more tired two hours after working out than you were before you started, you did too much."

Avoid brisk exercise when it's hot or humid outside -- especially during the first trimester when crucial cell and organ development takes place. Instead, opt for a workout tape or a trip to the gym instead.

Stick with the program. Regular exercise will reap the most rewards; spurts of activity followed by inactivity put strain on your muscles and joints. ACOG say that working out every day is best, but three times a week will do.