A transfer of a day-6 blastocyst is completely viable. We will consider it as good quality. It is true that we normally try to prioritize day-5 blastocysts because they have reached the blastocyst stage earlier, but there is no reason why we shouldn’t use these embryos for a transfer.
It’s important to note that while day-6 embryos may develop a bit slower, they can still be of high quality and have a good chance of implantation. The timing of development doesn’t necessarily impact their potential for success, and it’s not uncommon for day-6 embryos to lead to successful pregnancies.
Yes, depending on the case, for example, on Day five we have more than one embryo and we can select one and sometimes it is better to wait till day 6 to see how embryos development is going on. There are pregnancies with transfers on Day 6.
It is unusual to transfer a day 6 embryo, to be honest. But they can be transferred. There is not much difference between day 5 and day 6. The more you progress with growing an embryo in the lab, the higher the chance that the embryo hatches from the zona pellucida, from the shell. And so as long as the embryologist and the lab are happy with the quality of the embryo, then I wouldn’t see any problem in transferring at day 6. But it very much depends on the policy of the clinic, and there have been some pregnancies on day 6 as well. But the standard practice is usually to transfer a day 5 embryo.
Of course, if we have two blastocysts, two hatching blastocysts (one of them day 5 and the other day 6 blastocyst), we will start with day 5 blastocyst. If we have only one embryo that is day 6 blastocyst, we will transfer it. So it’s always a little bit lower chances with day 6 blastocyst, but it’s not a lost case if you only have a day 6 blastocyst. Sometimes the development of the embryo is slower. If we don’t have blastocyst on day 5, we wait till day 6 for the blastocyst to develop. If it is well-developed, the blastocyst is good for transfer.
Proper growth and proper development of the embryo is the most important thing for us. On the 3rd day, the embryo must reach 6-8 cells and on the 5th day, it must reach the blastocyst stage. If a day-5 embryo does not reach the blastocyst stage but it is in the early blastocyst stage, we wait one more day until the 6th day to evaluate it and check if the embryo will reach the blastocyst stage or not. If the embryo reaches the blastocyst stage even on the 6th day, in this case, if the embryo’s physical structure is good and satisfying, in this case, we freeze them, and we will be able to use them.
There is only one problem. In the uterus, there is a concept we call an “implantation window” which means the uterus can’t accept an embryo until the 5th day after the follicle rupture. After the 5th day, even if you transfer the best embryo, the implantation window will be closed, and the uterus will not accept the embryo. On the 6th day, if you put the 6th-day embryo, in this case, you can never get pregnant. In this case, if we have a 6th-day embryo, we prepare the frozen cycle for the patient and on the 5th day, we unfreeze the 5th-day embryo and transfer the 6th-day embryo on the 5th day of the endometrium.
The answer is yes. We transfer a day-6 blastocyst. There are two situations in which we can encounter this. One is when there was a transfer on day 5, however, it was a difficult transfer and we have to cancel it on that day. We wait until the next day and then we freeze. So this is when you have day 6 embryos. The other situation is when we have slow progress of an embryo. Let’s say, from day 2 to 3 or from 3 to 5. We notice that there is slower than usual development of the embryos. For example, on day 3 instead of being like 8 cells, there are still like 6 cells. And then on day 5, blastulation is still not fully complete. We just give them the benefit of the doubt and then push another day. So another day is to make sure that the blastocysts have expanded and we have a good sort of blastocysts on day 6. And then it’s very normal to transfer the embryo. If it even hatches a bit, it’s not considered as something bad but it’s only helping with the implantation potential. So the short answer is: yes, we transfer day-6 blastocysts.
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.
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.
