Pitfalls in prenatal diagnosis: cytogenetic analysis in amniocytes fails to detect mosaic r(12)

Objective To review the accuracy of a prenatal diagnosis of a missed chromosomal mosaicism in amniotic fluid cell cultures and to see whether adapting the Dutch guidelines would have made any difference to the outcome in this case. Method Metaphases, obtained from cultured amniocytes and peripheral blood lymphocytes, were analyzed with different results. The amniocyte cultures were then reanalyzed and the risk of missing this mosaicism in prenatal analysis was assessed. Results The prenatal tests performed according to the Dutch guidelines showed a normal female karyotype, but more extensive postnatal analysis revealed a ring chromosome in 50% of the child's lymphocytes. Reanalysis of the original amniocytes confirmed the normal diagnosis, but when more cells from the same and other colonies were analyzed, the ring chromosome was detected. Conclusion The chance of missing such a supernumerary ring mosaicism is very low (about 2% in our case). Given its very rare occurrence and the low chance of it being missed if the existing Dutch guidelines are followed, adapting the number of cells or colonies to be examined for all prenatal diagnoses does not appear to be justified. Copyright © 2003 John Wiley & Sons, Ltd.