Ok, so when I am able to actually fall asleep I usually wind up on my back, which I know I am not supposed to but when am I really supposed to stop that? I've never been much of a back sleeper, but it seems that I find that position most comfortable.
I heard 16 weeks is the magic "quit sleeping on your back!" number. I've also read in multiple places that when you wake up on your back, it's your body saying "Hey! Roll over!" Try getting a pretty squishy/flat pillow and put in under half of your back on one side to angle you off being flat on your back. Sometimes my hips & shoulders get tired of having all my weight on them all the time so this seems to help me a ton and I can tell it takes the pressure off. It's like halfway between being on your back or side.
The only thing that happens if you sleep on your back is that the baby will eventually start to put pressure on you and cut off YOUR blood supply... when that happens you will automatically wake up. So, basically you will just keep waking up several times a night, but it doesn't really pose any danger to you or your baby. How do I know this? Because I sleep on my back for my entire pregnancy. It doesn't matter how many times I roll onto my side - I always wake up an hour or two later on my back.
I am just now finding I don't like lying on my back. I really want to, but when I do, after about 90 seconds I start feeling sort of dizzy/nauseous. So I'm constantly flipping sides. Ugh.