RESUMEN
The results of screening for the common aneuploidies involving chromosomes 13, 18, 21, X and Y by florescent in-situ hybridisation (FISH) in direct preparations from 100 chorionic villus samples from pregnancies between 10 and 20 weeks' gestation are reported. Samples prepared using routine methods and analysed with commercially available probes, accurately detected 12 cases of fetal aneuploidy, all referred because of developmental abnormality. Three of the four cases where chromosome abnormality was detected in cultured villi but not by the direct fluorescence in situ hybridisation (FISH) assay, were due to confined placental mosaicism. No chromosomal anomalies were found in the 20 low risk cases where the referral reason was a familial single gene disorder. We conclude that the FISH assay with commercial probes may act as an accurate and less labour intensive alternative to direct chromosome analysis of chorionic villus samples. In cytogenetically low risk cases its use can obtain a result within the time needed for DNA analysis and avoid the need to set up cultures.