You will notice an effect more on you at first then on bub, especially as you get further along. When you lie on your back the weight of the uterus compresses some major blood vessels that return blood from the lower half of your body to your heart, it will/may make you feel lightheaded, dizzy, and nauseous. If you are worried about rolling onto your back during the night, stick a pillow behind you to wake you up when you roll. It may also make lying on your side more comfortable.