Recently the population-based large data set of the Hungarian Congenital Abnormality Registry showed an increase in the recorded total (birth + fetal) prevalence rate of informative offspring with Down syndrome. This finding was checked and confirmed in the completed data set due to the field study including all cytogenetic labs and prenatal diagnostic centres. The previous birth prevalence of 1.17 per 100 in the 1970s increased to 1.50 per 1000 between 1989 and 1999 with a maximum 1.77 in 1992. The completeness of the Registry was only 65.9% between 1970 and 1999, i.e. 30 years. One-third of trisomy 21 have not been karyotyped, however, recently this figure was about 10%. The major explanation of this increase is the significantly higher proportion of prenatally diagnosed fetuses with Down syndrome and an increasing part of women over 35. In addition better ascertainment may also contribute to this increase, but new etiological factors cannot also be excluded.
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