Blastocyst Culture

Day 2 or Day 5 embryos. What is the difference? IVF success rates have improved steadily in recent years but some people are still puzzled why success rates for embryo transfer are not higher still.

There are many different reasons why an embryo may not successfully implant in the uterus. The commonest reason, however, is because the embryo was not healthy or genetically normal. It is a biological fact of life that any couple will produce quite a significant proportion of abnormal embryos, regardless of whether this is with natural conception, or in an IVF cycle. In an IVF cycle, it may not be immediately apparent to us at the time of embryo selection and transfer that these embryos are not normal. At the present time, we assess embryos predominantly on the basis of the cell number and the appearance of the embryo, what we call morphology. We have very good data to show that the better the morphology of the embryo, the better the chance that this embryo will grow into a baby. It certainly does not automatically follow, however, that an embryo which appears very good (good structure and good cell number) is healthy and genetically normal and therefore able to successfully implant. Each year we continue to search for new and better ways to try and determine which embryo is most likely to grow into a baby when placed in the uterus. At the present time, however, we rely predominantly on cell number and morphology to do this.

At my clinic, Melbourne IVF we have traditionally performed the majority of our embryo transfers on Day 2, forty-eight hours after the time of fertilization. An alternative to this is to perform the transfer on day 5, when the embryo is known as a blastocyst. The theory behind blastocyst culture is that the longer a group of embryos are cultured in the laboratory, the more likely it will be that weaker embryos will fail to develop normally. It is very important to understand, therefore, that a significantly smaller number of embryos will be present by day 5 than might have been present on day 2. Occasionally no embryos survive at all. The chance of a day 5 embryo successfully implanting,however, is likely to be higher when compared to a Day 2 embryo.

Are there any “down-sides” to blastocyst culture? Why does Melbourne IVF not routinely use blastocyst culture? There are two reasons:

• If we could be absolutely certain that all the embryos that failed to develop between day 2 and day 5 were not normal or healthy and could not form into a baby, then clearly the decision to use blastocyst culture every time would be very easy. We cannot be sure about this, however. We believe that it is very possible that some day 2 embryos, which might have been able to implant and form a baby, will not culture successfully to day 5.

• By day 5 the embryo has become much more complex, both structurally and genetically. Blastocysts do not cope as well with the embryo freezing process. It is certainly true that techniques for blastocyst freezing have greatly improved in recent years. There is no doubt, however, that day 2 embryos cope better with this process. It is important to remember that, at Melbourne IVF, more than 50% of our babies come from frozen and thawed embryos.
  

As you can see, there are pros and cons to a decision to use blastocyst culture. To some extent, with blastocyst culture you choose to put all your eggs in one basket* (if you will pardon the awful pun). You may start with a good number of apparently good embryos on day 2. By day 5 you will have many fewer embryos (and possibly none!) but those which have survived may well have a better chance of implantation. Those blastocysts which are not implanted, however, may have a poorer chance of surviving the freeze and thaw process.

There is nothing new about blastocyst culture. It has been around for a long time. It is gaining more publicity, however, and it is important that you are aware of this important topic. I do not use this as a routine technique but I certainly use it, and I am happy for you to proceed with this if it is your wish. I would be very happy to discuss this important topic with you.

* Our best success rates come with the fresh embryo transfer, whether it is a Day 2 or a day 5 embryo.

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Extended culture of human embyos