Blastocyt is a day 5 embryo. It consists of an outer trophectoderm and an inner cell mass. It is at this stage of the embryo that it gets implanted in the uterus when conception occurs naturally. Only good quality embryos reach this stage when incubated in the laboratory after activation of embryonic genome. Thus when a blastocyst is transferred into the well prepared uterus during the window of implantation, it will have an implantation potential of 50-60%. Fewer blastocysts can be transferred, while a higher chance of conception compared to 8-cell embryos can be guaranteed. Hence multiple pregnancy rate is also reduced.