Researchers at the University of Cincinnati have found significant differences in the prevalence of Down syndrome between Hispanic and non-Hispanic whites as well as distinct racial differences in the use of prenatal genetic testing to diagnose Down syndrome.
Their findings are published in the Jan. 15 issue of the American Journal of Epidemiology and are based on an analysis of data collected by the California Birth Defects Monitoring Program and the Genetic Disease Branch of the California Department of Health Services.
"There are very few databases in the country that attempt to collect 100 percent of the data on birth defects. California, which is the population I chose, actively ascertains that data," said UC doctoral student Jennifer Bishop, explaining why the analysis should provide a very accurate assessment of the true prevalence of Down syndrome in the population.
With the assistance of UC biology professor Carl Huether, UC mathematics professor James Deddens and California researchers Claudine Torfs and Fred Lorey, Bishop analyzed data from 1989 through 1991. The data included both information on live births and prenatal diagnoses of Down syndrome. That's important, because many Down syndrome fetuses are spontaneously or electively aborted and would be missed by researchers looking only at live births.
"We really wanted to get as close a measurement as possible of the true prevalence in the population, because prenatal counselors often use these rates when they're trying to predict the outcome for older mothers," said Bishop.
Overall, Bishop found that the total prevalence of Down syndrome was slightly higher than figures previously reported in the literature. Apparently, that figure reflects the gradually increasing age of the American population.
The biggest surprise came when Bishop looked for differences among ethnic groups and races. "There seems to be a different type of pattern in Hispanic population," said Bishop. "The prevalence among younger mothers was higher in the Hispanic than in the other races and lower in the older maternal ages."
This does not mean that young Hispanic mothers have a greater risk than older Hispanic mothers. It simply means that when the researchers compared younger Hispanic mothers with younger white mothers, prevalence rates were higher for the Hispanic mothers.
Bishop looked at five-year age brackets, and for all age groups under 40, the risk of Down syndrome was significantly higher for Hispanics than for whites. For mothers over 40, the prevalence rates were significantly lower for Hispanics than for whites. However, the researchers still cannot explain why the difference occurred.
"What we found doesn't necessarily indicate a genetic difference," said Bishop. "It could be a socio-economic difference, a nutritional difference or an ascertainment difference. Did we really have all the cases?"
The other clear racial difference showed up when the researchers compared the prevalence of Down syndrome among live births and the adjusted prevalence of Down syndrome which included aborted fetuses. Whites showed a 46 percent decrease in Down syndrome births due to prenatal diagnosis compared with only a 10 percent reduction among Hispanics.
The researchers say the difference probably reflects several factors, including the availability of prenatal diagnostic services, the cost of those services, and differing personal beliefs on elective abortions.
Huether has found similar differences in previous studies comparing blacks and whites and mothers from different geographic regions.