Doctors recommend that you avoid sleeping on your back after your first trimester because it causes you to unintentionally focus your weight on a major vein that moves blood to the heart from your feet and legs, which in turn forces the heart to work harder. But doctors also tend to be quick to tell you not to worry about it if you wake up and find that you've rolled onto your back. Changing positions while you snooze is normal, and losing sleep over whether you're in the right position doesn't do anyone any good.
That said, it's recommended that you train yourself to sleep on your side—ideally your left one because the position improves blood flow throughout your body, including to your uterus and fetus. Using body and regular pillows as sleep props is a great way to help you get and stay as comfortable as possible during the coming months.