A blastocyst is an embryo that has been developed in the laboratory for five or six days after insemination, in contrast to conventional IVF which involves transferring embryos to the womb two to three days after egg collection and insemination (this is referred to as a Day 2 or 3 transfer).
With a blastocyst transfer, the embryo has advanced to the five or 6-day stage. This means the embryo has divided many more times into many more cells over this period. Blastocysts have a very thin outer shell thus potentially increasing the chances of implantation into the uterine cavity. Most of the blastocyst contains a fluid cavity and it is possible to see the cells which will become the baby and those which will make up the placenta. 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.
If you have experienced recurrent implantation failure, blastocyst extended culture gives us the opportunity to examine embryos over a longer period. If embryos arrest or become fragmented, this can help us identify any potential problems.
When a single embryo transfer is specifically required, typically for anyone who has a history of multiple pregnancy or a uterine issue, blastocyst transfer may be a particularly useful option.
Your consultant or embryologist will advise on whether blastocyst transfer is a good option for you as an individual.
We know that at least 50% (or higher in women over 40) of embryos are not viable, and many of these embryos stop developing before the blastocyst stage. A large proportion may have an incorrect number of chromosomes and it is believed that those embryos that failed to develop to the blastocyst would not, in any event, have established a pregnancy. Where there are large numbers of good quality embryos available at the blastocyst stage these can be frozen. We have seen excellent post thaw survival and pregnancy rates with frozen blastocysts.
Research has shown that conditions in the womb may be more optimal for a blastocyst than a day 2/3 embryo as there are slightly differing conditions in the fallopian tube and the womb on day 2/3.
Data suggests that blastocyst transfer can increase the chances of a live birth. Please bear in mind that every patient is an individual and must be considered independently.
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 their development.
The main benefit of the blastocyst transfer approach is the ability to select different embryos in terms of their quality and implantation potential. It is essential to understand that the extended culture process doesn't enhance an embryos quality per se, it is a way of choosing the most viable from a group of embryos.