In the context of the EUROCAT study data on selected congenital malformatio
ns and chromosome aberrations were collected from the Canton of: Zurich (19
88-1997). It was found that the major proportion of severe and early malfor
mations, such as anencephalus and holoprosencephaly were detected prenatall
y; for oral clefts and meningomyeloceles this was not the case, at any rate
in regard to isolated (non-syndromic) malformations. However, if these def
ects occur in combination with a chromosome aberration, the likelihood of s
uch a case being registered is higher For the same reason, i.e. due to abno
rmal ultrasound findings and intrauterine growth retardation, trisomies 13
and 18 were more often detected prenatally than trisomy 21.