I slept on my back and on my right side during both pregnancies out of habit. I always have to fall asleep on my right side, and eventually end up on my back.. Just sleep how you are comfortable. With DD1 no one ever mentioned not laying on the back after the first trimester (I did regular yoga twice weekly w/her including the night I went into labor) so it was news to me when it was mentioned at every pre natal yoga class and on this board.