20 weeks pregnant sleeping flat on the back, the fetus generally will not lack of oxygen. 20 weeks pregnant in the middle of pregnancy, at this time the fetus in the uterine cavity has a larger space to move, and has the amniotic fluid package, pregnant women in the usual sleeping situation, generally will not cause the fetus to appear hypoxia symptoms. If pregnant women in late pregnancy, long time choose to lie down the way to sleep, the abdominal wall may press the fetus, and then cause the fetus to appear intrauterine hypoxia, affecting the normal development of the fetus. When pregnant women in late pregnancy, try to choose the left side of the sleeping position, can increase the blood flow and oxygen supply to the placenta, so as to avoid intrauterine hypoxia of the fetus, and help the health and development of the fetus.