Chronology of three consecutive mitotic events in human preimplantation embryos was examined by time-lapse imaging. In zygotes producing well-formed and pregnancy-yielding expanded blastocysts, uniform time-patterning of cleavage clusters (c) and interphases (i) was revealed: i2= 11±1, i3= 15±1, i4= 23±1 h/c2= 15±5, c3= 40±10, c4= 55±15 min. Oppositely, shortened or prolonged durations of one or more cell cycles were strongly predictive of poor implantation and development. Furthermore, trichotomic mitosis was discovered in 17% of cases-zygotes cleaved into 3 blastomeres and 2-cell embryos into 5-6 cells (instead of normal 2 and 4). During conventional clinical assessment, such embryos are indistinguishable from normal, often considered just-in-course of the next cell cycle. Only detailed time-lapse monitoring paced at 10-minute intervals had proven all these embryos to be absolutely unviable, even in rare cases when they reduced their hypercellularity to normal cell counts via cell-cell fusion. Overall, we demonstrate that timelapse embryo cleavage rating (ECR) as a standalone diagnostic procedure allows for effective identification of viable early embryos with 90% specificity, while elimination of good-looking but unviable embryos can be assumed with a specificity of