In my opinion, a day 5 transfer is better than with a day 3 transfer, and we routinely do blastocyst transfers for all our patients. A day 5 transfer allows us to select the best growing embryos , and we can then choose the ones which have the best chance of implanting. However, in some clinics the pregnancy rate for day 3 embryos is better , and I think the only possible explanation for this could be if the lab itself is not very good . They may not be very good at growing embryos in vitro , and if they try to do an extended culture, their suboptimal lab conditions maybe causing the embryos to arrest in vitro.
If a day 3 embryo is going to become a baby after putting it back in the uterus, then in a good laboratory, it will become a blastocyst in vitro as well. There are no two ways about this - a good IVF lab has enough confidence in its culture conditions in being able to grow embryos to Day 5. If the embryo has the competence of becoming a baby in utero after transfer, it obviously has the biological competence to become a blastocyst in the incubator as well, if it is handled with care and respect in vitro. After all, it couldn’t become a baby in the uterus without becoming a blastocyst first, and in a good IVF lab, the conditions which the lab provides are as good as in vivo conditions.
However, if the lab is not very good, then perhaps the extended in vitro culture causes progressive damage to the embryo , from which it cannot recover. The longer they try to keep the embryo in vitro, the more the damage they cause, perhaps as a result of toxins; or because they are not being able to maintain the temperature or pH very well; or because they handle the embryos roughly. These progressive in vitro insults cause cumulative damage over a period of time. In these labs, the cumulative damage over 3 days is not enough to cause resilient embryos to become non viable , so that if they do manage to put the embryo back in the uterus in 3 days, this embryo can recover from the insult it has suffered in the IVF lab . Once it's safe inside the uterus, where it’s no longer suffering the in vitro insults which a poor quality IVF lab subjects it to, it can recover , and happily continue to grow into a blastocyst and then a baby.
However, in the same poor quality lab tries to grow the embryo to Day 5, that same embryo keeps on getting insulted , so that it may not have enough energy to form a blastocyst, and it arrests in vitro. Or sometimes, the additional 2 days of insults in vitro prevent the blastocyst from continuing to develop in utero after the transfer.
This is why having a high quality lab makes such a big difference to the outcome of IVF treatment.
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I think this is a gross over generalization. I did IVF with the renowned Advanced Fertility (IL, US) and due to only having five eggs fertilize, of seven mature, my RE recommended a three day transfer, even though they prefer a five day and have better results with them. I still got pregnant from that cycle, just as I did from my five day frozen cycle with them. Many clinics operate this way to my knowledge.
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