Down screening is also known as Down’s syndrome screening. Early Down’s syndrome screening is usually done between 11 and 13 weeks of pregnancy, and mid-term Down’s syndrome screening is done between 15 and 20 weeks of pregnancy. Down’s screening is a test in which blood is drawn from a pregnant woman to measure the concentration of alpha-fetoprotein, chorionic gonadotropin, and free estriol in the mother’s serum, which is then combined with the mother’s age, weight, and week of gestation to determine the risk factor for Down’s syndrome in the fetus. Down’s screening is categorized into early Down’s screening and intermediate Down’s screening. Early Down’s screening is usually performed between 11 and 13 weeks of pregnancy, and intermediate Down’s screening is performed between 15 and 20 weeks of pregnancy. A low-risk result indicates that the fetus has a relatively small chance of developing Down’s syndrome, while a critical risk or high-risk result indicates that there is a possibility that the fetus may develop Down’s syndrome, but it is not absolute. If the Down’s syndrome screening result is abnormal, further non-invasive DNA amniocentesis can be done to determine whether to continue the pregnancy through the test.