American Journal of Epidemiology Vol. 145, No. 2: 134-147
Copyright © 1997 by The Johns Hopkins University School of Hygiene and Public Health
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Epidemiologic Study of Down Syndrome in a Racially Diverse California Population, 1989–1991
1Department of Biological Sciences, University of Cincinnati Cincinnati, OH
2California Birth Defects Monitoring Program Emeryville, CA
3Genetic Disease Branch, California Department of Health Services Berkeley, CA
4Department of Mathematical Sciences, University of Cincinnati Cincinnati, OH
Reprint requests to Dr. Carl A. Huether, Department of Biological Sciences, 821A Rieveschl Hall (ML 006), University of Clncinnati, Clncinnati, OH 45221-0006.
The prevalence of Down syndrome was studied among all live births occurring between 1989 and 1991 in the California counties monitored by the California Birth Defects Monitoring Program. Objectives of this study were: 1) to calculate adjusted prevalence rates and quinquennial maternal age-specific risk rates of Down syndrome after adjusting for elective abortion of prenatally diagnosed fetuses; 2) to estimate the impact of prenatal diagnosis and subsequent elective abortion of affected fetuses on the observed prevalence of Down syndrome; and 3) to examine sex ratios among livebom infants and fetuses with- Down syndrome. The racial/ethnic diversity and large size of the population allowed the data to be stratified into five racial categories—Hispanics, whites, Asians, blacks, and others. For the period 1989–1991, the observed prevalence of Down syndrome was 1.13 per 1,000 live births, and the adjusted total prevalence, which took into account the termination of affected pregnancies following prenatal diagnosis, was 1.53 per 1,000 live births. In a comparison of quinquennial maternal age-specific risk rates of Down syndrome by race, Hispanics and whites were the only groups with rates that differed significantly from each other, with Hispanics exhibiting higher rates at maternal ages under 40 years. The overall reduction in live births with Down syndrome in 1989–1991 that could be attributed to prenatal diagnosis and elective abortion of affected fetuses was 25.8%, with a 49.1% reduction being observed at maternal ages 
35 years. In 1990–1991, Hispanics had the lowest overall reduction (10.0%), while whites had the highest reduction (46.3%). The male: female ratios among liveboms with Down syndrome were significantly higher than those among all live births, and race had a significant association with sex ratios in both cases and controls. These findings indicate that prenatal diagnosis and elective termination of affected pregnancies has had a substantial impact in reducing the number of liveboms with Down syndrome in the monitored California counties. The effect was greatest for whites and least for Hispanics, with results indicating considerable variation in the use of prenatal diagnostic services among racial/ethnic groups. Estimates of adjusted total prevalence and reduction in live births with Down syndrome in this study should be considered minimal because of some underascertainment of prenatally diagnosed cases. Am J Epidemiol 1997; 145: 134–47.
age factors; Down syndrome; maternal age; prenatal diagnosis; racial stocks; sex ratio
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