Trends in live birth prevalence of down syndrome in the Northern Netherlands 1987-96: the impact of screening and prenatal diagnosis.

In the Northern Netherlands, we examined the live birth prevalence of Down syndrome (DS) and the impact of maternal serum screening (MSS) and prenatal cytogenetic diagnosis (PCD) during the period 1987-96. In this period the live birth prevalence, based on the maternal age distribution and the age specific risk of delivering a child with DS was expected to increase from 1.26 in 1987 to 1.62 in 1996. The introduction of MSS in 1991 made PCD available to women of all ages. Nevertheless, the utilization of PCD remained very stable. In 1991, 4.7% of pregnant women underwent a diagnostic test. In 1996 this percentage was 6.4%. As a result of MSS and PCD, the live birth prevalence of DS was 19% lower than expected (p<0.01). Despite utilization of PCD based on opting-in and a discouraging government policy regarding the offer of MSS, the percentage of DS cases detected by PCD increased from of 17% during the period 1987-90 to 27% in the period 1991-96 when MSS was available. The percentages have been corrected for spontaneous pregnancy loss. From a medical and financial point of view, MSS was the most cost-effective indication for PCD. However, the potential of reducing the birth prevalence of DS is limited by the low utilization of MSS and PCD by pregnant women.