This is what I found on the Internet : Don't be alarmed if you go to sleep on your side and wake up flat on your back. It happens all the time and shouldn't have any serious adverse effect on your baby's health. Just shift your position and go back to sleep.
My due date is June 6 and I been sleeping in my back my whole pregnancy and thanks god my baby is healthy I always try to sleep on a side but like you said I wake up on my back, my point is I don't think there is a big problem sleeping on your back.
I always sleep on my back but when my leg falls asleep I tuck my covers underneath my back so I'm kind of tilted, it helps a lot.
FTM! (27 weeks) I am having the same problem sleeping at night! I tend to wake up on my back & I get nervous as well hoping she is not getting less oxygen. I'm starting to sleep elevated it helps a little but if your really concerned call your doctor!
Try a body pillow its u shaped and huge will help you sleep and keep in one place