During the second trimester of pregnancy, experts recommend sleeping on your side. Sleeping on your belly at 16 weeks pregnant might be a little uncomfortable, and experts believe that lying on your back can increase pressure on the blood vessel that returns blood to your heart.
Sleeping on your left side can improve your circulation, allowing better blood flow to the fetus and to your uterus and kidneys.
Try using a pillow between your knees and another under your abdomen for more comfort.