Answer from: Raúl Olivares, MD
Transferring or freezing an embryo on day 6 is something that should not impact the outcome of the treatment. The embryos become blastocyst on day 5, but sometimes they can become blastocyst in the morning, midday, in the evening, or sometimes even on day 6, in the morning. As long as the quality is good, the day 6 blastocyst should offer very similar pregnancy rates to a day 5 blastocyst.
It is not also uncommon that when we do PGS, for example, and we test the embryos genetically, we may get day 5 embryos that are all abnormal, and the only normal one is the embryo that has been biopsied on day 6.
Transferring or freezing day 5 or day 6 is okay. It’s true that once we reach day 7, the chances drop dramatically, and probably it’s only worth carrying out these transfers when you don’t have any blastocyst on day 5 or day 6. Day 6 blastocyst should always be considered as a good embryo for achieving a pregnancy.
Answer from: Harry Karpouzis, MD, MRCOG, DIUE
We can have embryos that divide slower. We may have some embryos that on day 5 haven’t got to their full potential. If we leave them to develop for one more day, till day 6, they can become very good grade embryos, survive day 6 and give very good chances of success.
Still, there is an issue here because if we are talking about fresh transfers, there is data that says if we do the transfer on day 6 and not on day 5, the chances of success are getting smaller. But the reason behind that is not the embryo itself. It’s the coordination with the endometrium and the implantation window. Many times when we see that an embryo is not good enough on day 5, we want to leave it to develop till day 6 to get to its full potential, and we prefer to freeze it. In this case, we can still get very good chances of success in a frozen transfer. Even when we have frozen embryos on day 6, there are some studies that compare transfer on the 5th day of progesterone with the 6th day of progesterone. We have found out that even if the embryo has been frozen on day 6, the chances of success with the frozen embryo transfer are higher than when an embryo is transferred after 5 days of progesterone.
Even with frozen embryo transfer, even if we have a day 6 embryo frozen, we prefer to transfer it on the 5th day of progesterone rather than on the 6th. Despite that, day 6 embryos can be very good embryos and can give good chances of success if they are used correctly.
Is day 6 blastocyst used for transfer? In what situation day 6 embryos may be used for transfer?
Are day 6 blastocysts good? What are the chances of 6-day blastocyst implantation? How long does it take a 6-day blastocyst to implant?