The impact of maternal serum screening on the birth prevalence of Down's syndrome and the use of amniocentesis and chorionic villus sampling in SouthAustralia

Objectives To describe the impact of maternal serum screening on the birth prevalence of Down's syndrome and on the use of amniocentesis and chorionic villus sampling in South Australia. Design A descriptive population-based study. Setting South Australia (population 1.48 million persons; approximately 20, 000 births per year). Participants Women who had births or terminations of pregnancy with Down's syndrome in 1982-1996, women who had maternal serum screening in 1991-1996, amniocentesis or chorionic villus sampling in 1986-1996. Methods Analysis of data from multiple sources on maternal serum screening, amniocentesis and chorionic villus sampling, births and terminations of pr egnancy. Main outcome measures Total prevalence and birth prevalence of Down's syndr ome each year in 1982-1996; proportion of pregnant women using maternal ser um screening in 1991-1996, and proportion using amniocentesis and chorionic villus sampling by indication in 1986-1996, by age group. Results Use of maternal serum screening for Down's syndrome increased from 17% when introduced in 1991 to 76% of women who gave birth in 1996. Between 1982 and 1986 and 1996, terminations of pregnancy for fetal Down's syndrom e increased from 71% to 75% and the birth prevalence of Down's syndrome fel l by 60% from 1.05 to 0.42 per 1,000 births, against the background of an i ncrease in total prevalence due to increasing maternal age. The use of amni ocentesis increased from 5.8% in 1991 to 10.1% in 1996 mainly due to the in crease among women younger than 35 years with maternal serum screening as t he main reason. The increasing chorionic villus sampling rate among younger women stabilised at 0.4%, while the rate among older women decreased from 11.0% to 7.4%. Conclusions The introduction of maternal serum screening in South Australia has resulted in increased use of any prenatal testing for Down's syndrome from about 7% (mainly older women having amniocentesis or chorionic villus sampling) to 84% of women (about 8% having direct amniocentesis or chorioni c villus sampling and 76% having maternal serum screening first). This has resulted in a significant fall in the birth prevalence of Down's syndrome, maternal serum screening was the first indication of Down's syndrome for ab out half the terminations of pregnancy for Down's syndrome in 1993-1996, in cluding three quarters of those in younger women.