The study looked at how well fast-cleaving embryos on day 3 develop compared to slower ones. They analyzed over 4000 embryos from 513 cycles and found that embryos with 9 to 11 cells had slightly lower chances of becoming healthy embryos than those with 8 cells. However, embryos with more than 11 cells had similar chances. Slow-cleaving embryos had lower chances. Fast-cleaving embryos and 8-cell embryos had similar success rates for live births. Embryos with more than 11 cells were just as likely to result in a live birth as 8-cell embryos.