The second Down’s syndrome screening is usually done in the middle of pregnancy, at 16-21 weeks of pregnancy is more appropriate. The second Down’s syndrome screening is a re-testing method mainly for the first Down’s syndrome screening that appears to have a high risk or is unclear. The test measures the concentration of alpha-fetoprotein, chorionic gonadotropin and free estriol in the mother’s serum, as well as the woman’s age, week of pregnancy, weight and due date, in order to predict the risk of congenital disorders such as trisomy 21 in the fetus. When the screening has a high risk problem, further non-invasive DNA or amniocentesis is needed for further examination to prevent the birth of malformed fetuses. This disease seriously affects intelligence and there is no treatment, only prenatal screening and prenatal diagnosis to rule it out.