Trends in Down's syndrome prevalence in California, 1983-1988.

Trends in the prevalence of Down's syndrome in livebirths were examined in California from 1983 to 1988, a time period when prenatal screening and subsequent termination of Down's syndrome pregnancies among women under 35 years of age was increasing. In the population-based ascertainment areas of the California Birth Defects Monitoring Program (CBDMP), 1058 infants with Down's syndrome were identified, giving a crude prevalence of 1.03 (95% confidence interval [CI]: 0.97, 1.09). The risk ratio for 1988 compared with 1983, after adjusting for maternal age and other confounders, was 0.98 (95% CI: 0.78, 1.23), showing that there was no change in livebirth prevalence over this period. The data were also examined for mothers under 35 years of age to determine whether increasing use of prenatal screening leading to termination masked an increase in prevalence. A modest increase was seen.