Trisomy 21 is clinically known as trisomy 21, which is caused by chromosomal abnormalities. It is now believed that trisomy 21 is mostly caused by the non-separation of chromosomes during meiosis when the pregnant woman is too old at the time of conception, and is associated with both sperm and eggs. In addition, physical rays, chemical pollution, viral infections and genetic factors may lead to chromosomal disorders. 1, causative factors and high prevalence: 1, physical factors: one or both spouses work in a radioactive environment for a long time, or the working environment is contaminated by radiation; 2, chemical factors: before and after gestation, one or both spouses, taking drugs that can easily lead to fetal abnormalities, such as tetracycline, pesticides, etc.; 3, viral factors: before and after gestation, pregnant women have a history of influenza, rubella, measles viral hepatitis, etc. The chance of fetus suffering from trisomy 21 will be increased if there is a history of viral infection; 4, genetic factors: one or both spouses have chromosome abnormalities, such as chromosome balance translocation, the parents themselves can behave normally, but the offspring may also suffer from trisomy 21; 5, other factors: pregnant women have a history of habitual abortion, premature birth, stillbirth and other medical history, as well as older people, the chance of fetus suffering from trisomy 21 will be increased. The chances of the fetus suffering from trisomy 21 will increase. Symptoms: Trisomy 21 is a congenital developmental abnormality with mental retardation as its main manifestation. It is common to see mental retardation, physical retardation and special facial features, such as wide eye spacing, low nasal bridge, inability to open eyes, drooling and tongue sticking out of the mouth. Some patients are also prone to congenital heart disease and other malformations, and have low immunity during growth. As an adult, symptoms of Alzheimer’s disease often appear at the age of 30, causing inconvenience in life. Therefore, it is recommended that pregnant women be screened for Down’s syndrome early in pregnancy, and for high-risk pregnant women, amniotic fluid chromosome screening should be performed during pregnancy to reduce the number of children born with trisomy 21.