Embryos can be cultured for up to six days, until they become blastocysts. At this stage it may be easier to select the best quality embryo(s).
With blastocyst transfer, embryos are cultured in the laboratory to the blastocyst stage before they are transferred into the womb.
Blastocyst transfer is useful where single embryo transfer is specifically indicated (eg. previous history of multiple pregnancy, patient preference, uterine anomaly etc).
The goal of IVF/ICSI is to provide high quality embryos which are capable of continued development and result in live births.However, under standard IVF culture conditions, only about 25 to 60% of human embryos progress to the blastocyst stage after 5 days of culture.
This low rate of embryo development has 2 main causes:
- A less than optimal culture environment in the lab dish.
- The inherent “weakness” of human embryos.
One problem with this is that 2 to 3-day-old embryos are normally in the fallopian tubes, not in the uterus. The embryo gets to the uterus about 80 hours after ovulation.
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.
The transfer is done shortly before the time for actual invasion and implantation. Pregnancy rates and reduce risks for multiples
Transferring blastocysts following IVF also provides another potential benefit – reducing possibility for multiple pregnancyMany 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.
Some patients have large numbers of frozen embryos and it can difficult to know which have the best potential for pregnancy. An option in these cases is to thaw all embryos and culture through to the blastocyst stage to allow the best 1 or 2 embryos to be replaced based on development.
What are the cons of blastocyst transfer ?
Sometimes when patients opt for a blast transfer , they might have a lower yield of embryos . This is because some embryos may stop development at the 4- cell stage.
There is a remote chance that a single blastocyst can sometimes give rise to monozygotic twins.
Blastocyst transfer may not be suitable for older women.