A blastocyst is the stage the embryo reaches after 5 days in culture from the egg retrieval. During the three first days of development the embryos start to divide in separate cells and they are called cleavage embryos. It is only from day 3 to day 5 that the blastocyst is formed. The blastocyst is a much more complex structure than that of the cleavage embryo. The number of cells has increased to reach over a 100. Under the microscope it is even possible to distinguish the two different cell types that have started to develop. The first of these cell types is known as the Inner Cell Mass (ICM), this is earliest sign of fetal development and is the part of the blastocyst that develops into the baby. The second of the cell types is known as the trophectoderm or trophoblast. These are specialised cells which develop the placenta and are important in embryo implantation.
While the majority of fertilised eggs will develop into a three-day old embryo, only perhaps 40% of these embryos will develop into a blastocyst. Therefore, blastocysts are considered to be a more "select" group of embryos with a higher chance of pregnancy.
Sometimes it is not possible to select the best quality embryo(s) to transfer and the day 3 stage. This is because at this stage all the embryos can still look relatively similar and it is impossible to select which embryo is capable of forming a pregnancy over those which are unlikely to. In such circumstances there is a tremendous advantage to let the embryos develop on for another 48 hours and allow them to develop into blastocysts.
Subsequently by letting the embryos develop to the blastocyst stage the embryologist will be able to make a better choice of which embryo(s) to transfer back. We achieve higher pregnancy success in doing this because those embryos that manage to become blastocyst in the lab have a higher potential to implant in the womb. Accordingly, we advise that if embryos develop into good quality blastocysts we replace a fewer number. This is because a blastocysts has a much greater potential to form a pregnancy and therefore it is more likely that a multiple pregnancy will develop if we transfer more than one.
The transfer procedure for a blastocyst is identical to that for a normal embryo transfer. However, the key difference is that a blastocyst is implanted back into the womb five to six days following the egg collection whereas an embryo is implanted two to three days following egg collection.
Not every embryo is capable of forming a blastocyst. Therefore, there is the small possibility that the IVF cycle may not result in a transfer of any embryos if none of the three-day old embryos develop into blastocysts. We believe that if blastocysts development does not occur, it is unlikely that a pregnancy would not have developed if the embryos had been transferred at the 3-day old stage.
In practice however, it is very rare for no embryos to develop to a blastocyst. If the embryologists have any concerns about the development they will discuss the possibility of transferring the embryos back into the womb earlier and not letting them develop to blastocysts.