Down syndrome

Down syndrome (DS) is the most common genetic cause of intellectual disability, with an incidence of 1 in 792 live births, or about 5300 of the 4 million infants born yearly in the US.[3]de Graaf G, Buckley F, Skotko BG. Estimates of the live births, natural losses, and elective terminations with Down syndrome in the United States. Am J Med Genet A. 2015 Apr;167A(4):756-67. http://www.ncbi.nlm.nih.gov/pubmed/25822844?tool=bestpractice.com Population estimates suggest that, as of 2010, the prevalence of DS in the US was approximately 6.7 per 10,000 individuals.[4]de Graaf G, Buckley F, Skotko BG. Estimation of the number of people with Down syndrome in the United States. Genet Med. 2017 Apr;19(4):439-47. https://www.gimjournal.org/article/S1098-3600(21)02433-3/fulltext http://www.ncbi.nlm.nih.gov/pubmed/27608174?tool=bestpractice.com ​ The incidence of DS is similar worldwide, occurring in 1 in 800 births.[1]Bull MJ. Down syndrome. N Engl J Med. 2020 Jun 11;382(24):2344-52. http://www.ncbi.nlm.nih.gov/pubmed/32521135?tool=bestpractice.com

DS occurs in all ethnic groups, socioeconomic levels, and geographic regions. Standard trisomy 21, the presence of an extra chromosome 21, is caused by chromosome nondisjunction and occurs in 95% of cases. In about 4% to 5% of individuals with DS, the trisomy results from a chromosome translocation, and the remaining 1% of cases of DS are mosaic with a combination of typical and trisomic cells.[5]Hunter AGW. Down syndrome. In: Cassidy SB, Allanson JE, eds. Management of genetic syndromes, 2nd ed. Hoboken, NJ: Wiley-Liss; 2005:191-210.