Doctors use epidemiology (population analysis) to determine prevalence rates of diseases & conditions.
The rate of down's is about 1.53 per 1000 pregnancies. Maternal age is the most important risk factor for ds. But hispanics under 40 were more likely to have ds, & pacific rim islanders more than filipinos, far asians, europeans or africans. Authorsbishop j, et al. Journal am j epidemiol. 1997.