It is widely acknowledged that it is safe for expectant mothers to sleep on their backs during the first trimester of pregnancy. However, as the pregnancy progresses into the second and third trimesters, it is generally not recommended to sleep flat on your back for extended periods.
Sleeping on your back later in pregnancy can compress the vena cava, a large vein. The weight of your uterus may compress this vein when you lie on your back, which could lower blood flow to the heart and uterus. Because of the reduced blood flow, the infant may experience dizziness, dyspnea, and low oxygen levels.
Pregnant patients are often advised to sleep on their side, preferably the left side, as this can enhance blood flow to the heart, uterus, and foetus. To support their bodies and keep from rolling onto their backs while they sleep, some people find it comfortable to place pillows between their legs or behind their backs.