Human blastocyst morphological quality is significantly improved in embryos classified as fast on day 3 (≥10 cells), bringing into question current embryological dogma

Objective: To evaluate developmental potential of fast cleaving day 3 embryos. Design: Retrospective analysis. Setting: Academic reproductive center. Patient(s): Three thousand five hundred twenty-nine embryos. Intervention(s): Day 3 embryos were classified according to cell number: slow cleaving: : 6 cells, intermediate cleaving: 7-9 cells, and fast cleaving: 10 cells, and further evaluated on day 5. The preimplantation genetic diagnosis (PGD) results of 43 fast cleaving embryos were correlated to blastocyst formation. Clinical outcomes of transfers involving only fast cleaving embryos (n = 4) were evaluated. Main Outcome Measure(s): Blastocyst morphology correlated to day 3 blastomere number. Relationship between euploidy and blastocyst formation of fast cleaving embryos. Implantation, pregnancy (PR), and birth rates resulting from fast embryo transfers. Result(s): Blastocyst formation rate was significantly greater in the intermediate cleaving (72.7%) and fast cleaving (54.2%) groups when compared to the slow cleaving group (38%). Highest quality blastocysts were formed significantly more often in the fast cleaving group. Twenty fast cleaving embryos that underwent PGD, formed blastocysts, of which 45% (9/20) were diagnosed as euploid. Aneuploidy was diagnosed in 82.6% (19/23) of arrested embryos. A 50% implantation and 100% PR and birth rate were achieved with embryo transfers involving fast cleaving embryos. Conclusion(s): Fast cleaving embryos not only reach the blastocyst stage at a similar rate to intermediate cleaving embryos, but also exceed morphological quality criteria on day 5. Fast cleaving embryo transfers demonstrated a high clinical potential. (Fertil Steril (R) 2008;89:358-63. (c) 2008 by American Society for Reproductive Medicine.)