Isaac sleeps on his belly too. I dont worry about it as i know he will shift if he needs to. As soon as i put him to bed he rolls straight onto his belly (usually with his bum in the air, its so cute). I roll him on his side when i go to bed but he always ends up back on his belly.
Beth congrats on the preg hun, you are going to be 1 busy mumma!!! Though im sure you will love it.
Hope all is well with everyone xx