In the first trimester, there are no significant changes in the volume of the uterus and abdomen, so you can sleep in your usual position. This is safe even if your favorite position is on your stomach or back.
However, starting from the second trimester and until the end of pregnancy, the safest position during rest and sleep is on your side. Sleeping on the stomach is dangerous for the baby, and it becomes difficult for the mother. The supine position (lying horizontally with face and stomach facing up) is also not advised, because maternal cardiovascular parameters and fetal oxygenation can change, sometimes causing significant changes in the fetal heart rate. This is due to the increased pressure of the uterus on the inferior vena cava (large vein carrying blood from the lower body to the heart) in this position, which leads to a decrease in the total venous outflow. Several epidemiological studies currently show that a maternal sleeping position on the back can be a risk factor for stillbirth, as well as low birth weight and fetal growth restriction (FGR).