Down syndrome was the first intellectual disability described and is the most common genetic form of intellectual disability. Due to its frequency, the general-public is most aware of Down syndrome, and for many, it is the prototypical form of intellectual disability. Approximately 7000 infants are born in the United States each year with Down syndrome. In fact, Down syndrome accounts for approximately one-third of children in special education (Harris, 2005 p. 194).
The condition is not attributable to any behavioral activity of the parents or environmental etiologies, although some research on environment counters such statement. The probability that another child with Down syndrome will be born in a subsequent pregnancy is approximately 1%, regardless of maternal age. However, a number of risk factors have been investigated, including geographic location, race, ethnicity, season of birth and environmental hazards; however, none of these have been found relevant to affect the occurrence of the said condition (Harris, 2005 p. 194).
The incidence of trisomy 21 correlates strongly with increasing maternal age children of older mothers has much greater risk of having Down syndrome. Up to mid-30s, women have a constant risk of giving birth to a trisomic child of about one per 1000 live births. Incidence then increases sharply to 1 in 30 at age 45 years (Jobling, 2006). The estimated incidence of Down syndrome is between 1 in 800-1,200 live births. The risk of a mother having a second child with Down syndrome is 1% regardless of maternal age, unless the syndrome is associated with translocation of chromosome 21 (Rubin and Strayer, 2007 p. 190).