It is an inherited disorder characterized by a redundant third (or more) copy of the 21st chromosome. The extra chromosome material causes disturbances of development that result in a distinctive physical appearance (small round head, flat face, large tongue, epicanthic folds, simian lines), cluster of medical symptoms, an increased risk for various types of malformations, increased sensitivity to infections, increased risk of leukaemia, some degree of mental retardation, premature ageing and death. Cardiac malformations are very common; also intestinal malformations. Almost all Down syndrome children have problems with receptive and expressive language.
The chances of a woman having a baby with Down syndrome are one in 365 at 35 years, one in 109 at 40 years and one in 40 at 44 years of age. High paternal age is also implicated (some of the trisomies are also of paternal origin). There does appear to be an increasing incidence also for teenage mothers. Chromosomal translocation is also responsible for a small proportion of the genetic errors, about half of which are inherited. The rate of Down syndrome in a population is affected by the maternal age distribution and by prenatal diagnosis followed by elective abortion. The crude rates around the world are around 13 births per 10,000, but the variation is relatively large.