Pregnancy Over 40, Sleeping On Your Left Side May Increase Circulation To BabyI recall when I was pregnant, it was really hard to get comfortable. I found that laying on my left side was best. I even did this when I was in a recliner with the foot rest up. Actually, I found that sitting and even sometimes sleeping in a recliner was best because it helped with the nausea. When I I would lay down all the way, I think stomach acid would work its way up. As it turns out, laying on your left side may be the best when you are pregnant. This article explains more about the best sleep positions:
During the second half of pregnancy, avoid sleeping on your back, a position that puts the full weight of your uterus on your spine, back muscles, intestines, and the inferior vena cava (the vein that transports blood from your lower body to the heart). Back-sleeping can also put you at risk for backaches and hemorrhoids, inefficient digestion, and impaired breathing and circulation. Lying on your back in the second and third trimester can also cause changes in blood pressure. For some women, it can cause a drop in blood pressure that can make them feel dizzy; for others, it can cause an unwanted increase in blood pressure.
from: baby center