Expert answer: Russell Turk, obstetrician
If you typically sleep on your back, it's safe to continue doing so through the first trimester. But as the uterus gets heavier in the second trimester, it's best to choose another position.
When you lie on your back, your uterus presses on the vein that returns blood from your lower body to your heart. Lying on your back for an extended period of time could interfere with the flow of blood and nutrients to the placenta and your developing baby.
This is potentially unsafe, particularly if you have a condition like high blood pressure that affects how much oxygen and nutrients the baby is getting. If you have such a condition, your doctor will advise you to sleep on your side.
Don't be alarmed if you go to sleep on your side and wake up flat on your back. It happens all the time and shouldn't have any serious adverse effect on your baby's health.
Nonetheless, try to train yourself to sleep on your side or sitting propped up. If you need to, place a pillow between your legs, under your hip, or behind your back to make your sleep position more comfortable.
Expert answer: Catherine Lynch, ob-gyn
Once you get to the third trimester of pregnancy, I don't recommend lying flat on your back for any prolonged period of time. When you lie on your back, the increased weight of your uterus presses on the major vein that returns blood from your legs to your heart.
Of course, when you're sleeping you can't control what position you're in. If you wake up on your back, it's probably because your body was telling you to shift your position.
I recommend sleeping at an angle, tilted slightly to one side or the other. You can put a pillow between your legs to make that position more comfortable.