[Down syndrome in the northeastern Netherlands; expectations and reality 1981-1986].

Down syndrome is caused by presence in triplicate of (a part of) chromosome 21, usually on the basis of a standard trisomy-21. The risk of a child's having a standard trisomy-21 depends on the mother's age. On the basis of the maternal age distribution and age-specific risks, the expected number of neonates with Down syndrome born in 1981-1986 in the EUROCAT region North-East Netherlands is calculated. The number of neonates with Down syndrome registered with EUROCAT does not differ to a statistically significant extent from the expected number. The influence of the rising age-specific fertility in women over 28 years of age and the effect of the moving-up of the increasingly large birth cohorts 1945-1965 are discussed. If the uptake rate for prenatal cytogenetic diagnosis remains constant, these factors will in the next few years lead to an increase of the prevalence of Down syndrome among full-term infants.