What is the safest position for sleeping while pregnant?

By 12 weeks of pregnancy the uterus has become too large to remain completely in the pelvis. When the pregnant woman lies on her back, the uterus falls to rest on the vertebral column and the adjacent blood vessels. As the pregnancy progresses, when the woman lies in on her back, the large pregnant uterus consistently compresses on the venous system; the system in your body that returns blood back to the heart from the lower half of the body. This in turn causes the filling of the heart to be decreased and can decrease the amount of blood that the heart is able to pump to the body.

The maternal position, or the position of the mother's body during pregnancy, has significant effects on the blood flow throughout the body. The total output of blood from the heart and the amount of blood pumped with each beat is highest when the woman is lying on her side. The heart rate is lowest while lying on her side as well.

Also, blood flow out of the lower half of the body can be slower in pregnancy. This lack of flow can be attributed to obstruction of pelvic veins due again to the size of the uterus. The increase in pressure of the veins in the lower half of the body returns to normal if the pregnant woman lies on her side. For both of these reasons, the ideal position for sleeping while pregnant would be for the mother to lie on her side.