Monitoring trends in prenatal diagnosis of Down's syndrome in England and Wales, 1989-92.

The national register of chromosomal anomalies that lead to Down's syndrome has enabled the monitoring of change in prenatal diagnosis for this condition, and the factors which affect the change. The proportion of cases of cytogenetically diagnosed Down's syndrome in England and Wales detected prenatally rose to 46% in 1991-2 from 31% in 1988-9, a 1.5-fold increase (95% confidence interval 1.3 to 1.7). The increase was confined to mothers under 40 years and was due to the introduction of screening by maternal serum analysis and ultrasound. Over a quarter of affected pregnancies in women aged 25-29 were detected prenatally in 1991-2 compared with less than 10% in 1988-9. Analysis of the data showed regional differences in prenatal diagnosis rates, and in the length of time elapsing between the diagnostic test and termination of an affected pregnancy. An inexplicable finding was that this period varied with the sex of the fetus, being on average a day longer for females than for males.