['To analyze embryo morphology in relation to the corresponding chromosomal status, in order to evaluate the effects of aneuploidy over fragmentation, delayed cleavage, and arrested cleavage.', 'Retrospective study.', 'Reproductive Medicine Unit, Società Italiana di Studi di Medicina della Riproduzione, Bologna, Italy.', 'A total of 662 patients with a poor prognosis for pregnancy underwent 916 cycles of preimplantation genetic diagnosis for aneuploidy.', 'The chromosomes XY, 13, 15, 16, 18, 21, and 22 were analyzed in blastomeres biopsied from day 3 embryos.', 'Embryo morphology, chromosomal status of the analyzed blastomeres, and spreading and reanalysis of nontransferred embryos.', 'The incidence of chromosomal abnormalities was significantly higher in arrested or slow-cleaving embryos, and in embryos cleaving too fast, compared to embryos with eight cells at 62 hours after insemination. The presence of an uneven number of blastomeres or fragments scattered in the perivitelline space was associated with an increased incidence of chromosomal abnormalities.'] Are embryo morphology and development dependent on the chromosomal complement?