Lying flat on your back in the third trimester (28 weeks and beyond) decreases the circulation of blood to your heart and uterus (baby) making you feel light-headed. For this reason, you should not to lie flat on your back for long periods of time. However, as long as you are not feeling light-headed, there is no harm in lying on your back for short periods of time and there is no harm if you wake up and find that you have rolled onto your back in your sleep. If your doctor has determined that your baby is not growing well, you should make every effort to rest on you side while sleeping as laying flat may increase the risk of the baby dying.