IVF, In Vitro Fertilization with Blastocyst Culture and Day 5 Transfer
Blastocyst culture and day 5 embryo transfer for in vitro fertilization allows selection of the best quality embryos for transfer
Embryo quality is important with infertility
With blastocyst transfer, we transfer fewer embryos – reducing multiple pregnancy risks, and maintain high IVF success rates
Definition of a Blastocyst
An embryo that has developed for five to seven days after fertilization and has 2 distinct cell types and a central cavity filled with fluid (blastocoel cavity)
The cells in a blastocyst have just started to differentiate
The surface cells that surround the cavity (just under the outer shell) are called the trophectoderm and will later develop into the placenta
A more centrally located group of cells – the inner cell mass, will become the fetus
Blastocyst Development
The blastocyst usually forms on day 5 as fluid builds within the compacted morula
A healthy blastocyst often begins hatching from its outer shell, called the zona pellucida between day 5 to day 7 after fertilization
Within 24 hours after hatching, embryo implantation after IVF (or a “natural” pregnancy) begins as the embryo invades into the uterine lining
The blastocyst releases HCG hormone (the pregnancy test hormone) which leaks into the mother’s blood as the embryo implants
Blastocyst Formation Rate
The goal of in vitro fertilization and embryo culture 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
The 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
Therefore, in the past embryos were transferred to the uterus after 2 or 3 days
Advantages of blastocyst transfer for IVF
One problem with this is that 2 to 3-day-old embryos are normally n 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
Maintain pregnancy rates and reduce risks for multiples
Transferring blastocysts following IVF also provides another potential benefit – reducing possibility for multiple pregnancies.
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 a high order (triplets or higher) multiple pregnancies.
Extended in-vitro culture systems
In the past, it was difficult to get high-quality blastocysts with in vitro culture systems – unless “feeder” cells were utilized – called coculture.
However, since 1998 more advanced culture media have been commercially available that (if used properly) can yield high blastocyst formation rates.
Now blastocyst embryo transfer is a viable IVF treatment option for many couples
Disadvantages of blastocyst transfer
If the IVF clinic is proficient at blastocyst culture and has excellent quality control in the laboratory, there is potentially no disadvantage to day 5 blastocyst transfer.
However, if the culture environment is suboptimal, delayed embryo development and even embryonic arrest will occur in some cases.
Therefore, if the culture system and laboratory quality control are inconsistent – good results will not be obtained with extended culture to day 5.
Such programs will do better with day 3 transfers – putting back embryos earlier before they are “stressed” excessively by the weak culture environment.
Another possible disadvantage – some clinics charge more for day 5 transfer
