You should stop sleeping on your stomach or back by around 20 weeks of pregnancy. By this point, you may notice that sleeping either way — especially on your stomach — is pretty uncomfortable. The concern with sleeping on your back is that the enlarging uterus can compress the large veins in the back of your abdomen, which can in turn decrease the amount of blood returning to your heart. As a result, your blood pressure can drop, and you may feel lightheaded or nauseated. If your blood pressure drops, the blood flow to your uterus and baby may decrease. So, in general, after 20 weeks I recommend sleeping on your side.