Down's syndrome. Recent trends in the United States.

The crude incidence of Down's syndrome (DS) in the United States is currently about 1/1,000 births. Reduction in the proportion of births to women 35 years and older can account for a halving of the estimated percentage of DS births to this age group and a drop in the estimated crude incidence of DS from 1.33/1,000 births in 1960 to 0.99/1,000 births in 1978. Epidemiologic studies suggest that among women 35 years and older, the risk of having a child with DS has not changed. With the present distribution of maternal ages, prenatal diagnosis among women 35 years and older can result in no more than a 20% decrease in the crude incidence of DS. With continued use of prenatal diagnosis among older gravidas, upward of 80% of DS births will occur to younger mothers.