We asked two leading embryologists to break down the science behind Blastocyst Transfer.
A blastocyst is the name given to an embryo five to six days after fertilisation. Blastocysts look different to the ball of cells you see prior to the fifth day, explains Dr. Alan Thornhill, Scientific Director at the London Bridge Fertility, Gynecology and Genetics Centre.
On a blastocyst, you can see a clump of cells polarised at one side, which will become the fetus, as well as the cells that will become the placenta. In other words, differentiation has begun.
What is blastocyst transfer and how does it work
For a blastocyst transfer to take place, embryos are cultured in a laboratory incubator until they reach the blastocyst stage, then they are transferred to the womb. The procedure for blastocyst transfer is similar to a normal embryo transfer, the only difference being that the embryos have been given more time to develop in vitro (outside the body). This allows doctors to better select the embryos most likely to develop into a healthy baby but a blastocyst still needs to implant successfully in the womb for a woman to become pregnant. It s important to know that blastocyst transfer is not about making a better embryo, explains Dr. Thornhill, it s about better embryo selection. We re starting with a group of embryos and effectively putting them through a test outside the body, looking at which embryo does best in a culture system, and then transferring the embryo that appears to have the best chance of becoming a healthy baby into the womb. In the past, it was common practice to transfer every available embryo, but we now have rules to reduce the risk of multiple pregnancy. If we were allowed to transfer as many embryos as we wanted, selection wouldn t be an issue. But because we can t and shouldn’t transfer an unlimited number of embryos, selection has become more important.
What are the other benefits of blastocyst transfer?
Lots of people, particularly patients over 45, don t want twins, says Dr. Jennifer Rayward, director of ProcreaTec International Fertility Clinic in Madrid. Blastocyst offers the best chance of a successful single embryo transfer.
Dr. Rayward says that going to the blastocyst stage also helps when there are doubts about sperm quality. After the third day, we can see male factors being expressed, she says. If we ve been getting good quality day two embryos, but we re not getting to the blastocyst stage, the problem could be due to the man. Blastocyst helps us determine that. Another reason blastocyst transfer is so popular is that the receptivity of the womb in IVF cycles, particularly when using donor eggs, appears to be better after five days than after the second or third day. Most of the people who travel to our clinic from abroad are using egg donation because of Spanish law, and these people have already been through lots of fertility treatment. They want their very best chance, and their best chance is usually blastocyst. The main benefit is getting a baby quicker because, if it s suitable for you, blastocyst transfer stacks the odds in your favour, says Dr. Thornhill. If there are no genetic problems in the family, the woman has a good womb, there are no major problems with the man and you can make a good blastocyst, then you have an extremely good chance of getting pregnant. If you have good quality blastocysts left over, and access to a good freezing programme, you have a good chance in a frozen cycle too, he adds.
Who should have blastocyst transfer?
Because blastocyst transfer improves the chances of a successful single embryo transfer, it is particularly useful for younger women with a good prognosis for IVF treatment. A doctor may also suggest trying blastocyst transfer if you have produced good quality embryos in a previous IVF cycle, but they failed to implant in the womb.
It is survival of the fittest, so there are criteria applied at the clinic in order to get to the blastocyst stage, says Dr. Rayward. On day three, we d want to have at least five good embryos in order to go to blastocyst, because otherwise we re risking having no embryos left by the fifth day. If we got to day three, and you only had two good quality embryos, we wouldn t want a patient to take any risks by waiting. In that scenario, it s better to do the transfer at that stage, without going to blastocyst. You have a better chance if you can produce a healthy blastocyst, but there are no guarantees you will.
What are the drawbacks of the treatment?
Of the fertilised eggs that you start with, only around 50% make it to blastocyst, Dr Rayward explains. Then you need to think about cost, as blastocyst transfer is also more expensive. It involves more work for the embryologist and you have to have an excellent lab to get to day five. Additional costs vary from clinic to clinic, but Dr. Thornhill suggests that you can expect to pay an extra 200- 600 in the UK. Dr. Thornhill also says that blastocyst transfer may not be for patients who want their treatment to be as natural as possible.
The longer something is in culture, the more risks there are, however small, he says. Everything is well-monitored in a laboratory, but there is slowly accumulating evidence that keeping an embryo in culture for longer can alter gene expression. Lots of things alter gene expression its part of natural development so this could be a normal variation, but until longer-term studies have been completed, it s something we re yet to determine. If someone s embryos don t succeed in the culture system, that doesn’t necessarily mean they will never achieve a pregnancy, adds Dr. Thornhill. Embryos are meant to be in the body, and until a culture system is developed that is better than the womb, we have to be careful not to dismiss patients just because we can t get them to the blastocyst stage.
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ProcreaTec International Fertility Clinic in Madrid