Baby can lay on the cord cutting off his or her oxygen.
If you relax on your back make sure you are elevated or propped up with a couple of pillows
Whenever I sleep, I always catch myself laying on my back a lot, and still have a healthy baby boy. Doctor's just don't recommend it because as your uterus grows and you lay on your back there could be a chance of pinching a blood vein and cause problems. Just try laying on your sides.