Trisomy 21, or Down syndrome, is also known as congenital stupidity. It is the most common chromosomal abnormality in children with typical facial features, mental retardation, and some combined with severe organ malformations. As a result, prenatal screening for Down’s syndrome is widely promoted nationwide and consists of a brief serum marker test (“Down screening”) and ultrasound. A “high risk” screening result indicates that the fetus is at higher risk for Down syndrome than the general population and requires prenatal diagnosis. The method of prenatal diagnosis relies on amniocentesis, where amniotic fluid is extracted and karyotyped to determine whether the fetus is Down’s syndrome. Amniocentesis is not acceptable for some pregnant women because of the risk of amniotic cavity infection, fetal damage and miscarriage. In recent years, the discovery of fetal cells in the peripheral blood of pregnant women and fetal free DNA, as well as the research of new detection methods, have provided some new detection methods for Down’s syndrome. These non-invasive tests provide new options for pregnant women, but the following issues need to be noted when making decisions: 1. The incidence of Down syndrome increases significantly with age, so for pregnant women over 35 years of age, it is recommended that they undergo a direct prenatal diagnosis instead of a “Down screening” blood draw. It is emphasized that the overall risk of chromosomal abnormalities increases with age, and is not limited to trisomy 21, or Down’s syndrome. There are hundreds of chromosomal disorders alone, but there is no good method to check and predict them, and “Down screening” can only screen for the risk of trisomy 21 and trisomy 18, but cannot indicate the risk of other chromosomal disorders. 3. Amniocentesis and karyotyping are still the most valuable tests for diagnosing chromosomal abnormalities and the only methods that can be used as a basis for confirming the diagnosis. The comprehensiveness (hundreds of chromosomal diseases can be detected at the same time) and accuracy of the test cannot be replaced by any indirect methods. 4. Fetuses with Down syndrome have intellectual abnormalities, but not all fetuses with intellectual abnormalities are Down syndrome, and there is no reciprocal relationship between the two.