When you're pregnant, getting comfortable at night is no easy feat. After all, your back is aching, your legs are cramping, and your belly is threatening to take over the bed. On top of all that, you can add yet another nighttime inconvenience: figuring out which position is the safest for sleeping.

For obvious reasons, sleeping on your stomach isn't a good idea after the fifth month of pregnancy. And it's so uncomfortable that most women stop doing it much earlier, says Vera Stucky, M.D., an obstetrician at the University of California, San Diego Medical Center.

But sleeping flat on your back isn't the answer, either: Experts generally advise women to avoid this position after about the 16th week. "If you sleep on your back, the enlarged uterus presses against the inferior vena cava, the vein that returns blood from the lower body to the heart," Stucky explains. This can cause low blood pressure, exacerbate existing kidney conditions and hypertension, and contribute to the development of hemorrhoids, varicose veins, and edema.

In rare cases, sleeping on your back can even prevent the baby from getting all of the oxygen and nutrients it needs. Usually, however, the symptoms of low blood pressure  -- nausea, shortness of breath, dizziness, and clamminess  -- will rouse a snoozing mom-to-be long before her fetus is endangered. And nature itself may provide a second line of defense. "If the baby gets uncomfortable, it will start kicking and wake up the mother so she can change position," says George Verrilli, M.D., an obstetrician in private practice in Rhinebeck, NY.

So how should you get those precious zzz's? The safest bet is to lie on your side  -- preferably your left side, which allows for maximum blood flow  -- the next time sleep beckons. And if you wake up in the "wrong" position, simply roll over and dream of the day when that maneuver won't be a major undertaking.

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