Down syndrome or Down’s syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21.[2] It is usually associated with physical growth delays, mild to moderate intellectual disability, and characteristic facial features.[1] The average IQ of a young adult with Down syndrome is 50, equivalent to the mental ability of an eight- or nine-year-old child, but this can vary widely.[8]

The parents of the affected individual are usually genetically normal.[12] The probability increases from less than 0.1% in 20-year-old mothers to 3% in those of age 45.[3] The extra chromosome is believed to occur by chance, with no known behavioral activity or environmental factor that changes the probability.[13] Down syndrome can be identified during pregnancy by prenatal screening followed by diagnostic testing or after birth by direct observation and genetic testing.[5] Since the introduction of screening, Down syndrome pregnancies are often aborted.[14][15] Regular screening for health problems common in Down syndrome is recommended throughout the person’s life.[8]

There is no cure for Down syndrome.[16] Education and proper care have been shown to improve quality of life.[6] Some children with Down syndrome are educated in typical school classes, while others require more specialized education.[7] Some individuals with Down syndrome graduate from high school, and a few attend post-secondary education.[17] In adulthood, about 20% in the United States do paid work in some capacity,[18] with many requiring a sheltered work environment.[7] Support in financial and legal matters is often needed.[9] Life expectancy is around 50 to 60 years in the developed world with proper health care.[8][9]

Down syndrome is one of the most common chromosome abnormalities in humans.[8] It occurs in about 1 in 1,000 babies born each year.[1] In 2015, Down syndrome was present in 5.4 million individuals globally and resulted in 27,000 deaths, down from 43,000 deaths in 1990.[10][11][19] It is named after British doctor John Langdon Down, who fully described the syndrome in 1866.[20] Some aspects of the condition were described earlier by French psychiatrist Jean-Étienne Dominique Esquirol in 1838 and French physician Édouard Séguin in 1844.[21] The genetic cause of Down syndrome was discovered in 1959.[20]