It is possible to sleep flat on your back in the third month of pregnancy. The uterus is just beginning to protrude from the pelvis in the third trimester of pregnancy, and the uterine fundus can reach the pubic symphysis 2~3 transverse fingers, at this time the enlarged uterus can’t compress the inferior vena cava, so at this time you can lie down and sleep, pregnant women can also choose their own comfortable position to sleep, such as the left lateral position, the right lateral position and so on, as long as it doesn’t cause any discomfort. However, it is not recommended to sleep flat on your back in late pregnancy, so as not to increase the uterus to compress the inferior vena cava, resulting in a decrease in the volume of return blood and cardiac output, and the occurrence of supine hypotension, so it is recommended to take the left side of the lying position in late pregnancy, and if the left side of the lying position is uncomfortable, you can also choose the right side of the lying position. In late pregnancy, you should also strictly count the fetal movements, and go to the hospital on time for examination when there is a decrease in fetal movements.