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.
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