Preimplantation genetic diagnosis (PGD) is usually performed on cleavage stage embryos on day 3 post-insemination. Fluorescent in situ hybridization (FISH) has revealed four groups of chromosome patterns in embryos at this stage: uniformly normal, uniformly abnormal, mosaic and chaotic. Recently, some in vitro fertilization (IVF) clinics have started to perform blastocyst stage transfer. In blastocysts, conventional karyotyping has shown that all four groups of chromosome patterns are observed. In the present study, embryos were cultured to day 5 and were subject to a two-round multicolour FISH procedure for chromosome analysis to ensure almost every nucleus was examined. Probes for chromosomes X, Y and 18 were used in the first round and those for chromosomes 13 and 21 in the second round. Twenty arrested embryos (274 cells) and 19 blastocyst stage embryos (1272 cells) were analysed. Four arrested embryos and two blastocysts were uniformly diploid. The remaining 33 embryos were mosaic, including 17 blastocysts. Most of the blastocysts had a high proportion of diploid cells while in the arrested embryos, this proportion varied widely. For PGD, this high prevalence of mosaicism persisting to the blastocyst stage may pose problems similar to mosaicism in cleavage stage embryos.
Copyright 2000 John Wiley & Sons, Ltd.