COMPARATIVE EPIDEMIOLOGY OF DOWN-SYNDROME IN 2 UNITED-STATES POPULATIONS, 1970-1989

This study compared the epidemiology of Down syndrome over a 20-year period, 1970-1989, in two populations in which livebirths with Down syndrome were believed to be highly ascertained. One population was a 10-county region in southwest Ohio; the second was a five-county region in metropolitan Atlanta, Georgia. The major objectives were to 1) compare observed incidences of Down syndrome over the entire study period; 2) calculate expected incidences of Down syndrome for each population and compare these with observed incidences; 3) determine incidence trends throughout the period; and 4) examine the impact of prenatal diagnosis on the observed incidence of Down syndrome in each population. Excluding aborted fetuses with Down syndrome diagnosed prenatally, the observed incidence of Down syndrome, 0.98, was significantly lower than that expected, 1.27, within the Atlanta white data set. Once the observed data were adjusted to include aborted fetuses with Down syndrome (total incidence = 1.17), no statistically significant differences remained. No differences were found between observed and expected incidences of Down syndrome for whites in southwest Ohio or for other races in either population. Although uncorrected observed incidences were significantly different between the two populations of whites for the entire 20-year period, the differences again disappeared after the data were corrected to include aborted fetuses. No significant effect of prenatal diagnosis on those of other races was found in either population. Expected Down syndrome incidences rose from the late 1970s throughout the 1980s for both racial groups in Ohio and Atlanta, Georgia; however, because the termination of fetuses with Down syndrome increased over the decade, the observed incidence among whites in southwest Ohio remained at earlier levels and actually fell among whites in Atlanta. These results indicate the increasing importance of prenatal diagnosis and selective abortion in the epidemiology of Down syndrome and of correcting for it when comparing incidences between populations, even within the same time period. These data from 2 decades show that differences in both observed and expected incidences of Down syndrome exist between both populations and races, principally because of differences in demographic age structure and maternal age fertility rates and because of differential use of prenatal diagnosis and selective abortion.