During IVF treatment embryos are created within the IVF laboratory. When they are between the 2 day and 6 day old stage they are transferred back inside the womb (embryo transfer).
When this transfer is done on day 5 or 6 the embryo is an advanced structure called a blastocyst. We use blastocyst transfer for many couples, especially those with three or more good quality embryos on day 2 of their development. By watching embryo development longer the embryology scientists are better able to choose the higher quality embryos for transfer. This improves your chances of getting pregnant.