I sleep like that too but I now put a flat pillow under my waist/tummy area as it gives more support there for me. Its the comfiest for me! I thought it was ok to sleep on your tummy, until it just gets too uncomfortable, but I have a vague memory of being told if you can feel a huge amount of movement from baby then it means youre squashing baby and its better to move more onto your side! I have no idea how accurate that is though..