Down syndrome is a genetic disorder with a high incidence. The main cause of Down syndrome is an abnormality in chromosome 21 during division, resulting in a triploid next generation, often with a high incidence in older mothers. The main characteristics are: (1) External features are different from normal, with a wide distance between the eyes, a flattened nasal bridge, and upturned lateral corners of the eyes. (2) Low intelligence: IQ is usually about 20-50, and the distance between the two will increase day by day. (3) Developmental retardation: their bodies are abnormally weak, and a significant proportion of affected children also have damage to the heart and other organs, and in severe cases may have tetralogy of Fallot syndrome. (4) Late sexual and reproductive maturity, or even lack of fertility, female patients may have menstruation during puberty and may have offspring, but basically their offspring are children with 21 bodies. Some children with chimerism may be mentally impaired and have a face consistent with Down’s syndrome, but they are able to care for themselves and have no other behavioral or motor abnormalities. Since chromosomal disorders cannot be treated completely and have a very high disability rate, prenatal testing is necessary to detect any abnormalities and to treat them promptly, otherwise they can cause great damage to the family and the child.