The epidemiology of Down syndrome in four counties in Ireland 1981-1990

We conducted a descriptive epidemiological study of Down syndrome (DS) in the four Irish counties (Dublin, Kildare, Wicklow and Galway) covered by EUROCAT registries of congenital malformations for the years 1981-1990.

EUROCAT registries, which cover defined populations, use multiple sources for case ascertainment. All DS cases born between 1 January 1981 and 31 December 1990 to mothers resident in the four counties were identified. Crude birth prevalence rates and maternal age-standardized rates (SPRs) were calculated for each county and for each year in the study period. The prevalence of DS by maternal age grouped in five-year periods and the risk for each five-year group were also estimated.

The crude birth prevalence for the four counties was 18.5/10 000 for all births and 18.3 for live births. There was a fall in the total number of DS births over the decade, but less change in the crude birth prevalence owing to an increase in the proportion of mothers aged 35+. Galway had the highest crude birth prevalence of DS (23.5/10 000) but the SPR was within average for the four counties as a whole - 110.3, 95 per cent confidence interval (CI) 86.7-139. The risk of having a DS child increased 70-fold from 1:1841 at age 15-19 to 1:26 at 45 years or older. Nearly half of all DS cases (47.4 per cent) had at least one additional anomaly.

The parts of Ireland covered by EUROCAT have a high birth prevalence of DS births as compared with some other countries, but the maternal age-specific rates are not substantially different from those in large international studies.