Down screening refers to Down’s syndrome screening, including early Down’s syndrome screening and mid-term Down’s syndrome screening. Early Down’s syndrome screening is recommended to be performed at 11-13 weeks plus 6 days of pregnancy by ultrasound, while mid-term Down’s syndrome screening is recommended to be performed at 16-18 weeks of pregnancy by taking peripheral blood from the pregnant woman. Early Down’s screening refers to the ultrasound test to check the thickness of the fetal nuchal translucency, or NT test, which is recommended to be performed at 11-13 weeks plus 6 days of pregnancy, and based on the results of the test, the risk of having Down’s syndrome is determined. Mid-term Down’s syndrome screening is recommended between 16 and 18 weeks of pregnancy. Peripheral blood is drawn from the pregnant woman to check the concentration of serum alpha-fetoprotein, human chorionic gonadotropin and free estriol in the mother, etc., which is also combined with the mother’s age, body weight and week of pregnancy to determine whether the fetus has Down’s syndrome and neurological blood and tube defects, etc. If the test indicates that the fetus has Down’s syndrome, it is recommended that the test is performed at 11-13 weeks plus 6 days. If the Down’s syndrome screening test indicates a high risk, non-invasive DNA prenatal testing is recommended, and amniocentesis is required if necessary.