stb and all ladies - gt this from internet on sleeping position for preggies ladies - You may have heard that you should avoid lying on your back during your last two trimesters. Here's why: When you lie flat on your back, your uterus can partially compress a major vein (called the inferior vena cava) that returns blood from the lower half of your body to your heart. Lying on your side (or even tilted just a bit to the side), particularly your left side, prevents this compression and improves your blood flow. Try placing a pillow behind you or under one hip for comfort.