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

This study compared the epidemiology of Down syndrome over a 20-year p eriod, 1970-1989, in two populations in which livebirths with Down syn drome were believed to be highly ascertained. One population was a 10- county region in southwest Ohio; the second was a five-county region i n metropolitan Atlanta, Georgia. The major objectives were to 1) compa re 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 tre nds throughout the period; and 4) examine the impact of prenatal diagn osis on the observed incidence of Down syndrome in each population. Ex cluding aborted fetuses with Down syndrome diagnosed prenatally, the o bserved incidence of Down syndrome, 0.98, was significantly lower than that expected, 1.27, within the Atlanta white data set. Once the obse rved data were adjusted to include aborted fetuses with Down syndrome (total incidence = 1.17), no statistically significant differences rem ained. No differences were found between observed and expected inciden ces of Down syndrome for whites in southwest Ohio or for other races i n either population. Although uncorrected observed incidences were sig nificantly different between the two populations of whites for the ent ire 20-year period, the differences again disappeared after the data w ere corrected to include aborted fetuses. No significant effect of pre natal diagnosis on those of other races was found in either population . Expected Down syndrome incidences rose from the late 1970s throughou t the 1980s for both racial groups in Ohio and Atlanta, Georgia; howev er, because the termination of fetuses with Down syndrome increased ov er the decade, the observed incidence among whites in southwest Ohio r emained at earlier levels and actually fell among whites in Atlanta. T hese results indicate the increasing importance of prenatal diagnosis and selective abortion in the epidemiology of Down syndrome and of cor recting for it when comparing incidences between populations, even wit hin the same time period. These data from 2 decades show that differen ces in both observed and expected incidences of Down syndrome exist be tween both populations and races, principally because of differences i n demographic age structure and maternal age fertility rates and becau se of differential use of prenatal diagnosis and selective abortion.