Down syndrome, also known as congenital dysgenesis or trisomy 21, is a condition caused by the presence of an extra chromosome 21. Normal people have only two chromosomes 21, while people with Down syndrome have three chromosomes 21, so Down syndrome is also known as “trisomy 21”. The main manifestations of Down syndrome are severe mental retardation, inability to take care of themselves and congenital heart disease, leukemia, digestive tract abnormalities and other diseases. Because the cause of Down syndrome is not known, it cannot be effectively prevented or treated. Therefore, the birth of newborns with Down syndrome can only be prevented through prenatal diagnosis and selective abortion. This is why it is important to establish screening facilities and to provide universal screening for women during pregnancy (10-22 weeks). Down syndrome screening is divided into Phase I (10-13 weeks of gestation) screening and Phase II (14-22 weeks of gestation) screening. Phase I screening: refers to screening performed at 10-13 weeks of gestation and is a recently developed new method that focuses on fetal nuchal translucency-NT, maternal blood PAPP-A, and free b-hCG concentrations. Phase II screening: This is screening performed at weeks 14-22. This screening has a long history, starting in the 1980s, and more combinations of methods have been developed, currently the triple method (using maternal blood HCG, free E3, and AFP as markers) and the quad method (using maternal blood HCG, free E3, AFP, and inhibin A as markers) are more commonly used. . Family planning, eugenics, and improving the quality of the birth population is a basic national policy of China. Therefore, prenatal screening for Down’s syndrome is necessary as an important measure to prevent and reduce the birth of children with congenital defects.