You should sleep in the most comfortable position that you can. You will probably avoid sleeping on your stomach when you get later in your pregnancy due to comfort, but feel free to continue doing so if it is comfortable for you. Sleeping on your side is generally the best for blood flow, but do not worry if you wake up in the middle of the night and you are sleeping in a different position. Some women worry about blood flow issues sleeping on your back, but your body will wake you up before there are any issues regarding blood flow to the baby so do not stress.