The term ‘blastocyst’ refers to the human embryo 5 – 6 days after fertilisation.
The structure of the blastocyst is more complex than earlier embryo stages because along with an increase in cell number, the cells also become organised into 2 types, the trophectoderm, whose main role is in the implantation into the uterine lining and the inner cell mass which will give rise to the foetus itself.
Transferring embryos at the blastocyst stage provides a better co-ordination between the embryo and the uterus & hence higher chances of achieving a pregnancy. However the chances of having no embryos for transfer at all are also higher, so it is best decided by the embryologist & your IVF specialist to identify which (if any) of a group of embryos have the best potential to grow to the blastocyst stage in extended culture.