With all of the restrictions that come along with pregnancy, it's no surprise that sleep positioning is one of them. Given how hard it is to keep track of all of these restrictions, most pregnant women may never hear about the dangers of sleeping on your back after the first trimester.
As your baby grows, more and more pressure is put on your body to be able to withstand their weight. Laying in certain positions for long periods of time (i.e. sleeping) can be dangerous because there are vulnerable areas in which blood supply can be diminished if you and your baby aren't careful.
Specifically speaking in terms of the third trimester, when your baby is at his/her biggest before birth, research suggests that sleeping on your back can lead to a lower birth weight. Although the idea of having a smaller baby may sound appealing, especially for those who plan on delivering vaginally, the cause of this lower birth weight is due to lack of oxygen; something no mother wants for her baby.
Researchers at the University of Auckland studied more than 1,700 women who were in their third trimester of pregnancy. In comparing their sleeping positions to their babies' birth weights, they found that for babies whose mothers slept on their backs, they had an average decrease in birth weight of 5oz.
In addition to hindering proper growth, other studies have also made a connection with sleeping on your back and stillbirth due to the lack of oxygen. Researchers say that the safest position for any pregnant woman, no matter what trimester she's in, is on her side. In sleeping on your side, you shift your baby's weight away from the vena cava, "the large vein that returns blood from the head to the heart, and the aorta, which carries blood to the rest of the body," says DailyMail.
In examining other studies, scientists found that aside from the risk of stillbirth, the babies who had mothers who slept on their back during pregnancy were generally born at a healthy weight although lower than their side-sleeping counterparts. Mothers who slept on their left side compared to their right showed no significant effect on birth weight but that could be due to the sample size in the study. In any case, sleeping on your side proves to be the safest position during pregnancy.