[Down's syndrome in the Netherlands].

In 348 patients with Down syndrome (DS) born in the Netherlands in 1981 and 1982 chromosome studies had been performed before December 1st 1984. Prenatal chromosome studies had revealed 22 cases who could have been born in 1981 and 1982 taking into account a pregnancy duration of 40 weeks. DS rates are determined for five-yearly maternal age classes. These frequencies are compared with those from a reference group, consisting of a compilation of five epidemiological studies which are comparable in their way of presentation of the results and in the ascertainment of the DS cases. The most conspicuous finding in this comparison is that the frequency of DS among children of older mothers (greater than or equal to 40 years) in our study is relatively low. Therefore we presume that a number of patients born in the years 1981 and 1982, particularly from the maternal age groups 40-44 and greater than or equal to 45, are not (yet) referred for cytogenetic characterization. The great majority (+/- 90%) of the postnatally studied cases was presented within the first two months of life. In nearly 95% a standard trisomy 21 was detected and in about 5% a translocation or mosaicism.