The most common aneuploidies in prenatal diagnostics of the second trimenon are trisomies of chromosomes 13, 18, and 21 and gonosomal abnormalities. To detect these trisomies as quickly as possible after amniocentesis, besides using polymerase chain reaction, fluorescence in situ hybridization (FISH), applying corresponding centromeric or locus-specific probes, is the method of choice. Results of a rapid prenatal aneuploidy screening in uncultured amniocytes by using FISH are available within 24 hr or less. However, care has to be taken against possible pitfalls in connection with the commercially available probe sets and thus interpretation of results in general. Here, we explain how rapid prenatal aneuploidy screening is performed using the Food and Drug Administration-approved Aneu Vysion kit (Abbott/Vysis), and a review is given of drawbacks and opportunities of this method.