INTRODUCTION: In the course of the Down-screening protocol there are 
possibilities today for rapid diagnosis of aneuploidies among high-risk 
pregnancies identified by non-invasive screening tests, however, the diagnostic 
value of these molecular genetic tests are debated.
AIM OF THE STUDY: In this prospective study, data about the reliability of one 
of the rapid tests, namely; interphase fluorescence in situ hybridization 
(int-FISH) was to be gathered by the authors.
METHODS: For the period between May 2002 and September 2006 all of the 1279 
fetal sample were examined both with int-FISH and full karyotyping.
RESULTS: Extra or absent signal was detected in 47 cases (3.7%) (trisomy 21 in 
32, various other numerical abnormalities in 15 cases). All of these numerical 
aberrations were confirmed by metaphase analysis without false positivity or 
negativity. In 19 cases the finding of int-FISH was negative, however, full 
karyotyping disclosed abnormalities (in 12 of these 19 cases, the abnormality 
was balanced). Only 4 of the 1279 fetuses (0.3%) (3 small extra marker 
chromosomes, 1 de novo unbalanced translocation) were to be found, who would 
have been born with phenotypical abnormalities without metaphase analysis (2 of 
them had suspect ultrasound signs).
CONCLUSION: Although more analysis are needed, based on the results of this 
study it is to be concluded that rapid molecular genetic methods like int-FISH 
might be accepted as a diagnostic tests of fetal aneuploidy, if its use were 
restricted to high risk pregnancies identified by advanced maternal age and 
non-invasive maternal screening only. However, full karyotyping is needed in 
cases with familial translocation and abnormal 2nd trimester ultrasound signs.