In northwest European countries maternal age is increasing. This will lead to an increase of the prevalence of Down syndrome conceptuses. Meanwhile, the increased use of prenatal cytogenetic diagnosis (PCD) will lead to a decrease in the prevalence of Down syndrome among livebirths. We were interested to know what the result of these two opposite developments would be in the near future, and we describe here a model to quantify these processes and the resulting livebirth prevalence of Down syndrome. The model is demonstrated for The Netherlands from 1992 to 2001. The predicted livebirth prevalence for The Netherlands in 1992 is 1.36 per 1000. Demographic factors will cause an increase to 1.76 per 1000 in 2001 with present indications for PCD and a utilization ratio of 50%. An increase of the utilization ratio to 90% in 2001 will lead to a prevalence of 1.22 per 1000, a little less than the present prevalence. Alternative screening programs, including maternal serum screening, could lead to a further decrease of the livebirth prevalence. The model described here can be used for evaluation of the consequences of alternative forms of Down syndrome screening.
PIP: A model for predicting the prevalence of Down syndrome among future live births was applied to data from the Netherlands between 1992 and 2001. Concern has been raised about this issue since Down syndrome is associated with increased maternal age, and maternal age has been rising throughout northwestern Europe. The model was based on maternal age specific Down syndrome risk, local demographic factors, age distribution of females, age specific fertility rate, trends in age specific fertility rate, expected impact of prenatal diagnosis, type of screening, utilization ratio of screening program, and utilization ratio of selective abortion. Models applied 2 alternatives: 1) screening according to the present list of indications in the Netherlands, and 2) screening based on assessment of alpha fetoprotein, human gonadotrophin, and maternal age. The prenatal cytogenetic diagnosis (PCD) for Down syndrome in which the pregnancy was terminated was determined as 9.3% (registered cases in 19 European regions between 1980 and 1986. The PCD utilization ratio for all women aged 36 years and older was set at 50% in 1989, and 20% of nonusers were assumed to have a utilization ratio of 4.2% or 7.6%. In the Netherlands, all women aged 36 years and over have access to a diagnostic test for Down syndrome. Data on the maternal age specific risk of Down syndrome was taken from estimates by Cuckle et al. (based on empirical data from the US, the UK, Sweden, Australia, and Belgium). The assumption was that 98% of pregnancies with a prenatal diagnosis of trisomy 21 would be terminated for all screening programs. The prediction of Down syndrome for 1992 was estimated at 1.36/1000 at 50% PCD utilization and 1.76/1000 in 2001. For 90% utilization, Down syndrome prevalence would be 1.07/1000 in 2001. For 90% utilization, Down syndrome prevalence would be 1.07/1000 in 1992 and 1.22/1000 in 2001. If serum screening was used at 50% utilization, the prevalence would be 1.09/1000 in 1992 and 1.44/1000 in 2001. With 90% utilization and serum screening, the Down syndrome prevalence would be .63/1000 in 1992 and .75/1000 in 2001. The actual live birth prevalence of Down syndrome in the Netherlands was 1.16 between 1981 and 1986. Since discussion was devoted to current and possible future operations of screening programs.