Trisomy 21, also known as Down’s syndrome (DS), was the first chromosomal disorder identified and is the most common form of mental retardation caused by a single cause, with a prevalence of 1 in 600 to 1 in 800 live births. The relationship between maternal age and the development of DS has been established, with the risk of development increasing with maternal age. The clinical manifestations are varied, but the main features are threefold: peculiar facial features, mental retardation, reduced muscle tone, and physical retardation. The child is born with low birth weight and length and has low muscle tone. The head is small and round, the occipital region is flat, the face is round, the nasal bridge is low and flat, the lid fissures are thin and tilted outward and upward, the eye spacing is too wide, the inner canthus is pronounced, and the eyelashes are short and sparse and often tilted. The iris is sometimes whitish and often has cloudy crystals. The mouth is small with thick lips, the tongue is large and outstretched, salivating; the ears are small, with low ears and deformed auricles. The back of the neck is short and wide. The limbs are short, the hands are wide and fat; through the palm, the fingers are short, and the 5th finger is often inwardly curved. Short or missing middle finger joint. Treatment: Current treatment is limited to treating the symptoms, such as the use of certain drugs that promote brain cell metabolism and nutrition, and careful care and appropriate training for the patient. According to the specific situation of each child, appropriate medical and surgical treatment, if accompanied by other serious deformities can be considered surgical correction. Prevention: 1. The high incidence of Down’s syndrome in newborns is associated with the advanced age of the pregnant woman. In addition, family genetic factors, drug factors, chemical factors, infection factors and radiation factors may induce chromosomal aberrations. Before conception, avoid ionizing radiation, overdose of drugs and exposure to chemical substances and viral infections; pay attention to personal hygiene, maintain good living habits and pay attention to appropriate amount of physical exercise to enhance the body’s resistance ability. It is recommended that all pregnant women undergo prenatal maternal serum screening for Down’s syndrome, and prenatal cytogenetic diagnosis is recommended for high-risk groups such as pregnant women over 35 years old, those who have given birth to a child with Down’s syndrome, and those who have a positive screening test for chromosome 21 Robertson translocation or other karyotype abnormalities. 3. When the prenatal diagnosis confirms that the fetal karyotype is Down’s syndrome, the symptoms and prognosis should be explained to the pregnant woman and her family, and early termination of pregnancy is recommended.