Down syndrome occurs when an individual has three, rather than two, copies of the 21st chromosome. This additional genetic material alters the course of development and causes the characteristics associated with Down syndrome.
Down syndrome is the most commonly occurring chromosomal abnormality which affects people of all ages, races and economic levels. One in every 800 to 1,000 babies is born with Down syndrome.
The incidence of births of children with Down syndrome increases with the age of the mother. But due to higher fertility rates in younger women, 80 percent of children with Down syndrome are born to women under 35 years of age.