Alan - it was all over the news at one stage about the blood supply to the baby when lying on your back, that's why I asked my OB. He said it's very unlikely you'll harm your baby, because you'll experience the discomfort and move due to the lowered blood pressure. He answered me as if he's had to clarify this for all his patients coz they all asked him. hehe.

I also don't deliberately sleep on my back, but I find I'm waking up on my back. Perhaps I keep waking in order to change position? Anyway he said it's only of concern once your bigger. I guess there's more weight/pressure then.