A blastocyst is an embryo that has developed in culture for at least five days after fertilization and then is transferred in the female womb. This is a more physiological approach since it is known that under in-vivo conditions the embryos reach the uterine cavity around the end of the sixth day following fertilization. This also allows at times for natural selection of best quality embryo as only the better quality, genetically normal embryo will grow to this stage.
