Prenatal screening is a simple, feasible, non-invasive test to detect people whose offspring are at high risk of birth defects by prenatal screening for genetic disorders (such as Down’s syndrome) or congenital malformations (neural tube abnormalities, etc.) that have a high incidence and seriousness. Screening for suspicious cases and then confirming the diagnosis is an important step in preventing birth defects. Theoretically, the most ideal way to prevent the birth of defective fetuses is to do prenatal diagnosis of genetic diseases or congenital malformations directly for each fetus (i.e., amniotic fluid testing), but it is more complicated and expensive, and carries certain risks. Therefore, at present, a two-step test is used to take the mother’s blood for prenatal screening, prenatal screening to identify high-risk groups, and then various prenatal diagnostic methods (e.g. amniotic fluid chromosome test) to perform a confirmatory test for high-risk groups because the amniotic fluid contains fetal urine, etc. The accuracy rate is nearly 100%, which can achieve twice the result with half the effort. The prenatal screening test is not a confirmatory test (since the mother’s blood is drawn, it is impossible to know completely whether the fetus is abnormal or normal), and further confirmatory tests (amniotic fluid tests, etc.) are required for positive screening patients. A negative result indicates no increased risk and is not normal. Prenatal diagnosis (chorionic villus test or amniotic fluid test) is performed directly in the following cases: 1 maternal age at term (35 years), 2 previous pregnancy with chromosomal abnormalities, 3 chromosomal translocation in one spouse, chromosomal inversion in one spouse, 4 recurrent miscarriages in early pregnancy, 5 prenatal ultrasound abnormalities.