<span name="style_bold">Justification and aims: </span>In Costa Rica, the diagnosis of chromosomal fetal anomalies is realized only by conventional cytogenetic analysis of chromosomes obtained from cellular cultures. The waiting for the results can be long. Moreover with some frequency culture fails due to contamination or bad quality of the sample or they cannot be analyzed. This makes it necessary to have a simple and cheap methodology to obtain an accurate and rapid fetal diagnosis of trisomy 21, 18 or 13, in pregnancies of high genetic risk submitted to amniocentesis or cordocentesis. <span name="style_bold">Materials and methods: </span>Three multiplex PCRs were designed to amplify four different short tandem repeats of each of the chromosomes 21, 18 and 13. There were collected 93 samples (88 amniotic fluids and 5 fetal bloods), received in the laboratory between 2006 and 2008 with request ofor chromosomal analysis. The results of the quantitative fluorescent PCR were compared with the obtained cariotype of the same samples to stablish the accuracy demonstrate the reliability of the assay. <span name="style_bold">Results: </span>Accuracy of the assay was 100% and it was possible to obtain results within 48 hours. STRs analysis could be made in 77% of the samples where the cellular culture could not be done. <span name="style_bold">Conclusion: </span>The quantitative fluorescent PCR demonstrated to be a simple, accurate and rapid methodology, from what it might turn into a complementary tool of the chromosomal conventional analysis. The securing of rapid results in cases of antenatal diagnosis might diminish the period of anxiety parental for the waiting of the results, as well as to allow a better therapeutic management of the affected fetuses.The quantitative fluorescent PCR demonstrated to be a simple, accurate and rapid methodology, from what it might turn into a complementary tool of the chromosomal conventional analysis. The securing of rapid results in cases of antenatal diagnosis might diminish the period of anxiety parental for the waiting of the results, as well as to allow a better therapeutic management of the affected fetuses.

prenatal diagnosis; Costa Rica; trisomy; chromosome 13; chromosome 18; chromosome 21