As the fetus grows within the uterus, lying on your back is not recommended due to pressure on the inferior vena cava, a major vein that returns blood from the lower body to the heart. In addition, the increased pressure on the back and intestines can cause discomfort. Sleeping on the stomach during pregnancy also should be avoided, because of pressure on the fetus.
The best sleeping position for a pregnant woman is on her side, especially the left side, because it allows for maximum blood flow to the fetus and improves kidney function in the mother. Improved kidney flow helps to reduce any swelling. Placing a pillow between the knees can help a pregnant woman sleep more comfortably on her side.