A blastocyst is the embryo stage typically seen five days after fertilisation. A blastocyst has two areas which the embryologists assess, the outer surface of the blastocyst called the trophectoderm which becomes the placenta and an inner group of cells called the inner cell mass which develops into a baby.
If embryos look similar at earlier stages, culturing to this stage allows the embryologist to choose the best quality embryo(s) for transfer and can improve the chances of pregnancy. Not all embryos will reach the blastocyst stage and it may only be recommended when enough eggs have been fertilised. In addition to using extended culture as a selection tool, we know that in a natural pregnancy, a 3 day old embryo resides in the tubes and reaches the uterus on day 5 or 6. By transferring the embryo to the uterus on day five, the embryo is being returned to a more natural environment mimicking natural pregnancy. Blastocysts are suitable for genetic testing and can be successfully cryopreserved.