I think I read somewhere that it's comfortable to use pillows etc when sleeping because it can help to keep your own weight off the baby? So I'm not sure how sleeping on your stomach would be in terms of weight on the baby. But I'd say it'll soon be too uncomfortable anyway, and I think our body is pretty good at telling us what not to do. I know lying on my back now is just plain uncomfortable, which is a good thing, coz we're not sposed to do that.

I wouldn't worry too much.. we're probably not aware of half the positions we get into when we're asleep