Expected, prenatally discovered, and born cases of Down syndrome in Denmark during the period 1980-1998

In order to elucidate the consistency between generally used age-dependent risk values for Down syndrome (DS) and estimates of the probability of misc arriage in Down pregnancies we have compared expected numbers with estimate d numbers of births with DS in Denmark had no intervention at all been carr ied out. The expected numbers were calculated from the distribution of newb orn children according to maternal age combined with the age-related risk o f DS. The estimated numbers of children that actually would have been born without any intervention were estimated from observed numbers of cases of D S, i.e. the cases born plus - with corrections because of the high probabil ity of miscarriage in DS pregnancies - a proportion of those cases discover ed prenatally. The analysis was carried out separately for mothers aged 35 years or older and for younger mothers. We found a high degree of compatibi lity between expected and estimated numbers, probably with a minor underest imation of the expected values for the older mothers. The performance of DS screening in Denmark in the period tinder consideration (1980-1998) is dis cussed in relation to the figures presented. Despite the fact that 11.8% of all pregnancies were subjected to an invasive diagnostic procedure, only a bout 38% of all births with DS were prevented. This means that in the perio d 1990-1998, reluctance to accept serological screening has indirectly resu lted in the birth of almost 300 cases of DS in Denmark and at the same time the miscarriage of an unreasonable high number of normal fetuses. Copyrigh t (C) 2001 John Wiley & Sons, Ltd.