A Blastocyst is an embryo which has developed five to six days after fertilisation. It is at this stage that embryos implant into the uterus.
Using special culture medium, it is possible to allow embryos to grow in the laboratory for this length of time. Some embryos will successfully reach the Blastocyst stage which can then be selected for transfer. Research suggests that Blastocyst Embryo Transfer may improve pregnancy rates.