most books ive read say 27 so id go on that hun. and as for laying on your bk. i wake up every morning on my bk no matter how much i try and stop! but little one seems to have found a comfy spot curled round my belly button so he doesnt seem to mind! he sits in the exact position evry time im on my back! they do say the flow of oxygen is better if yo lay on you left side. but you cant control what you do when you asleep so i wouldnt worry! if your little one becomes uncomfortable with they way your sat they soon move! dont panic too much as theres not much you can do while he's/she's still in there!