Birth prevalence of Down's syndrome in England and Wales.

The natural birth prevalence of Down's syndrome for England and Wales in 1974-1987 (i.e., the birth prevalence in the absence of prenatal diagnosis and the induced abortion of affected pregnancies) was estimated by applying the maternal age-specific birth prevalence derived from epidemiological studies to the number of births in single-year age groups tabulated by the Office of Population Censuses and Surveys (OPCS). On average, the natural birth prevalence was 12.6 per 10,000 births and increased slightly from 12.2 to 13.2 per 10,000 births over the 14-year period. Using data on induced abortions carried out on account of Down's syndrome reported to OPCS under the statutory abortion notification scheme, 14 per cent of affected births were avoided by the induced abortion of affected pregnancies, so that the actual birth prevalence of Down's syndrome was estimated at 10.8 per 10,000 births. Using data on Down's syndrome births reported to OPCS under the voluntary congenital malformation notification scheme, the prevalence was 7.2 per 10,000 births, so only 67 per cent of the estimated number of affected births were, in fact, notified to the scheme.