It's a good idea to start getting used to a new sleep position now, since you shouldn't sleep on your back after 20 weeks. When you lie belly-up, the weight of your uterus can compress a major blood vessel, called the vena cava, disrupting blood flow to your baby and leaving you nauseated, dizzy, and short of breath. The best way to sleep during the second half of pregnancy is on your side. Some doctors recommend the left side over the right because the vena cava is located to right of your spine, so sleeping on your left side allows blood to flow more freely to your baby. However, this shouldn't make a big difference, and it's more important that you pick whichever side feels better. If you need help adjusting to side-sleeping, try slipping pillows between your knees and under your belly to make it more comfortable. Should you wake up on your back in the middle of the night, don't freak out. Your body would let you know if your baby was in any real danger of not getting enough oxygen -- you'd feel nauseated and breathless long before your baby would have a problem. Simply roll to your side and try to fall back asleep.