I heard that you shouldnt sleep on your back but i did through my pregnancy as it was the only way i was comfy and my baby was fine!
Yes definitely by second trimester, but you might get yourself used to other positions now...it sometimes takes awhile. My doc says that because it cuts off blood flow to that artery, you could actually pass out if you're on your back too long.
I was told that you should stop sleeping on your back in your second trimester. Although as my pregnancy has progressed that's the only position I'm comfortable sleeping in. I prop myself at an angle to relive pressure off my main artery in my body. Blood flow to you and your baby is crucial. Sleeping on your stomach doesn't seem right but I wasn't comfortable doing that.