Embryos formed as a result of fertilization are cultured in the laboratory for 5 days. In this time, the cells of the embryo rapidly divide and on day 5 it is referred to as a Blastocyst.
Transferring a 5-day-old embryo or Blastocyst into a woman's uterus may improve the chances of implantation and in turn, a successful pregnancy. It also reduces the risk of multiple pregnancies, which is a problem with Assisted Reproductive Techniques (ART) and which pose medical risks for both the mother and the infant along with financial and emotional considerations.
Advantages of blastocyst transfer for IVF
Embryo implantation process begins about 3 days later - after blastocyst formation and hatching out of the embryonic shell have occurred.
Therefore, if in vitro culture conditions are maximized so healthy blastocysts form at a high rate, then day 5 blastocyst embryo transfer can be done.
The uterine lining on day 5 should be receptive to the arriving embryo - this a more "natural" time for the embryos to be in the uterus. It is the same timing as with a natural pregnancy.
Many 2 or 3-day-old embryos do not have the capacity to become high quality blastocysts and make a viable pregnancy. However, on day two or three of culture we don't have methods to determine which embryos will be viable long-term, and which will soon arrest their development.
By culturing embryos to day 5 we will find that some of them have not become blastocysts - allowing us an opportunity to choose the most competent embryos for transfer.
We can then transfer fewer embryos and still obtain high pregnancy success rates - with very little risk for having high order (triplets or higher) multiple pregnancies.