To evaluate the impact of a large scale population screening programme on the birth incidence of Down's syndrome in the west of Scotland over a 12 month period. Biochemical screening for Down's syndrome using maternal serum α fetoprotein, chorionic gonadotrophin, and maternal age was offered to a pregnant population of 37 226 women in the west of Scotland between 1991 and 1992. The combined risk of Down's syndrome pregnancy was reported for each of the 30 084 women who opted for screening. Results — When a threshold risk of 1:220 was used 1523 women (5.1% of the screened population) were assigned to the high risk group, of whom 1070 (70%) proceeded to diagnostic ammiocentesis or midtrimester chorionic villus sampling. When multiple sources of ascertainment were used 37 Down's syndrome pregnancies were identified within the screened population, 26 (70%) of which were within the high risk group and 21 (57%) of which were prenatally diagnosed. In addition, three Down's syndrome pregnancies were diagnosed by first trimester chorionic villus sampling before biochemical screening. A further 10 Down's syndrome pregnancies were identified at birth, eight to women who had not had a screening test and two to women who had moved into the area, making a total of 50 Down's syndrome pregnancies in the whole pregnant population of 37 226. Thus the potential prenatal detection rate in the screened population was 70% (26/37), the actual prenatal detection rate in the screened population was 57% (21/37), and the overall prenatal detection rate in the total (screened and unscreened) population was 48% (24/50). Biochemical screening for Down's syndrome is practical and effective in routine clinical practice, enabling women to make an informed choice about prenatal diagnosis and providing better use of scarce resources when a suitable protocol is applied to the whole pregnant population. Its maximum potential for the reduction of the birth incidence of Down's syndrome is limited by incomplete uptake of screening and compliance with diagnostic testing in the high risk group.
Effect of non-invasive prenatal testing as a contingent approach on the indications for invasive prenatal diagnosis and prenatal detection rate of Down's syndrome