AbstractThe human blastocyst is key to understanding the aetiology of constitutional chromosome abnormalities in our species.ObjectivesTo investigate the range and incidence of chromosome abnormalities in a large series of human blastocysts, using classic cytogenetic techniques.MethodsUsing thymidine, cell division is synchronized in spare five‐to‐eight‐day‐old human blastocysts generated by IVF. A simple acetic acid disaggregation step produces discrete metaphases for G‐band analysis. Subsequent FISH analysis of both metaphase and interphase nuclei allows further exploration of an abnormality detected by G‐banding, including the investigation of any mosaicism.ResultsA total of 438 blastocysts have been prepared. Where analysis was possible, 3% appeared polyploid (mainly tetraploid), 29% were diploid : tetraploid mosaics and 68% were uniformly diploid. Abnormalities observed include triploidy, trisomy 16, trisomy 2, trisomy for unidentifiable D‐group chromosome, mosaic trisomy 3, and mosaic trisomy 3 and trisomy 7.ConclusionComparison of results with existing data from both first trimester pregnancies and cleavage stage embryos suggests significant loss of haploid and monosomic embryos, as well as loss of some trisomies, prior to the blastocyst stage. It appears that the general range and incidence of most main groups of constitutional abnormalities observed in the first trimester (including mosaic forms) are in place by the blastocyst stage. Copyright © 2002 John Wiley & Sons, Ltd.