Blastocyst culture simply means growing the embryos in the laboratory till it reaches a more advanced stage. Under IVF culture conditions, only about 60 to 70% of human embryos progress to the blastocyst stage after 5 days. Several eggs (oocytes) may have initially fertilised, but not all embryos will progress to the four-cell stage on Day 2 and to the eight-cell stage on Day 3 in culture, and even fewer will develop into blastocysts.
This selective embryo process can be viewed as survival of the fittest. The embryos that survive to this stage of development are more likely to be strong, healthy, and robust for implantation thus increase the chances for pregnancy.
Blastocyst transfer simply means that the blastocyst embryos are transferred to the woman’s uterus on day 5 in exactly the same way that early embryos are.