A blastocyst is an embryo that has advanced to the five-day stage which consists of 50-200 cells. Its structure is more complex than the earlier embryo stages as it organized itself into two types of cells, the trophectoderm (which gives rise to the placenta and extra-cellular tissue) and inner cell mass (which gives rise to the fetus). At the center, there is a blastocyst cavity that is filled with fluid (see video). This is a stage that the embryo must achieve before it can embed and implant into the uterine wall.
While the majority of fertilized eggs will develop into a two-day-old embryo (4 cells), only the strongest and healthiest embryos will develop into a blastocyst. Therefore, blastocysts are considered to be a more “select” group of embryos with a higher chance of implantation. In addition, blastocyst transfer provides better coordination between the embryo and the uterus by putting the embryo back in the right place (the uterus) at the right time (blastocyst stage).
Thus, pregnancy rates of five-day embryo transfer (blastocyst) are higher than that of two-day embryo transfer (4-cell-stage). The window of implantation of the blastocyst is five to six days after oocyte pick-up. However, blastocyst culture has a significantly increased risk of no embryo transfer due to the arrest of embryonic development before reaching the blastocyst stage.
Blastocyst transfer is not an option for all IVF patients. It is not suitable when there are less than three viable good quality embryos available. The technique is most successful with patients who are younger and have a large number of eggs available at retrieval. The indications of blastocyst culture are:
Repeated unsuccessful IVF cycles
Preimplantation genetic diagnosis (PGD)
Single embryo transfer to avoid multiple pregnancy