Blastocyst is the embryo on the 5th day of culture. At this point, it has about 120-200 cells and a cavity called blastocoele. Cell differentiation has already begun giving two kinds of cells the trophectoderm and the inner cell mass. The trophectoderm gives rise to the placenta and the inner cell mass forms the embryo.
A recent breakthrough for in vitro fertilisation is the use of blastocysts. A blastocyst would be implanted five to six days after the eggs had been fertilised. After five or six days it is much easier to determine which embryos will result in healthy live births. Knowing which embryos will succeed allows just one blastocyst to be implanted, cutting down dramatically on the health risk and expense of multiple births.
There are two advantages to Blastocyst Transfer. Firstly, the embryologists can choose the best blastocyst since it has been proved that embryos that reach the blastocyst stage have more chances to be genetically normal and therefore to implant giving a positive pregnancy test and a healthy baby. Additionally, the endometrium is more receptive for the embryos since the deleterious impact of the hormones has passed.
The only disadvantage of the method is that not all embryos reach the blastocyst stage and sometimes no embryo for the embryotransfer is available. So, even though the embryotransfer is cancelled the method is preferred to day 3 embryos embryotransfer since the couple can avoid a negative result after the transfer of an embryo that would not develop to later stages and therefore not implant.
Finally, the method concerns couples that have many embryos available and is a good evaluation test for the embryos, especially in cases with repeated failures in IVF programs and severe male factor infertility.