In the context of the EUROCAT study, data on selected congenital malformations and chromosome aberrations were collected from the Canton of Zurich (1988-1997). It was found that the major proportion of severe and early malformations, such as anencephalus and holoprosencephaly, were detected prenatally; for oral clefts and meningomyeloceles this was not the case, at any rate in regard to isolated (non-syndromic) malformations. However, if these defects occur in combination with a chromosome aberration, the likelihood of such a case being registered is higher. For the same reason, i.e. due to abnormal ultrasound findings and intrauterine growth retardation, trisomies 13 and 18 were more often detected prenatally than trisomy 21.