Down syndrome (DS) or Down's syndrome, also known as trisomy 21, is a chromosomal condition caused by the presence of all or part of a third copy of chromosome 21. Down syndrome is the most common chromosome abnormality in humans. It is typically associated with a delay in cognitive ability (mental retardation, or MR) and physical growth, and a particular set of facial ...view middle of the document...
 (MR has historically been defined as an IQ below 70.) A large proportion of individuals with Down syndrome have a severe degree of intellectual disability.
Down syndrome is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was clinically described earlier in the 19th century by Jean Etienne Dominique Esquirol in 1838 and Edouard Seguin in 1844. Down syndrome was identified as a chromosome 21 trisomy by Dr. Jérôme Lejeune in 1959. Down syndrome can be identified in a baby at birth, or even before birth by prenatal screening. Pregnancies with this diagnosis are often terminated.
The CDC estimates that about one of every 691 babies born in the United States each year is born with Down syndrome.
Many children with Down syndrome graduate from high school and can do paid work, and some participate in post-secondary education as well. Education and proper care has been shown to improve quality of life significantly