Down syndrome, the most frequently occurring chromosomal condition, appears in 1 of every 691 babies, or about 6,000 babies annually in the U.S. It typically occurs when, at the time of conception, an extra copy of chromosome 21 — or part of its long arm — is present in the egg or, less commonly, in the sperm. It is not known why this error occurs, and it has not been linked to any environmental or behavioral factors, either before or during pregnancy, but the risk is markedly increased with the age of the mother. At age 25, the risk is about 1 in 1,250 births; at age 40, it is 1 in 100. Because of higher fertility rates in younger women, 80 percent of children with Down syndrome are born to women under 35 years of age. Prenatal screening tests, such as the triple and quadruple screen blood tests, can accurately detect Down syndrome in about 70 percent of fetuses. Definitive prenatal diagnoses can be obtained with either chorionic villus sampling or amniocentesis.
Down syndrome is associated with approximately 50 physical and developmental characteristics. An individual with Down syndrome is likely to possess, to various degrees, some of these characteristics: mild to moderate intellectual disabilities; low muscle tone; an upward slant to the eyes; a flat facial profile; an enlarged tongue; and an increased risk of congenital heart defects, respiratory problems, and digestive tract obstruction. By age 40, nearly all people with Down syndrome show some neurological changes similar to those seen in Alzheimer’s disease, and most show cognitive decline by age 60.
Babies with Down syndrome develop much as typical children do but at a somewhat slower rate. Just as their peers do, they learn to sit, walk, talk, and toilet train. Early intervention programs can begin shortly after birth and can help foster an infant’s development.
Thanks to medical advances and a greater understanding of the potential of those with this condition, people with Down syndrome have been able to have longer and fuller lives. They are being educated in their neighborhood schools, participating in community activities, and finding rewarding employment and relationships.
Although there is no cure for Down syndrome or means of preventing it, scientists are moving closer to understanding the role that the genes on chromosome 21 play in a person’s development. There are several mouse models of Down syndrome that are allowing scientists to focus on molecular factors important in the condition. Once this mystery is understood, researchers hope to decode the biochemical processes that occur in Down syndrome and learn how to treat or cure this disorder.