Choosing Your Day of Transfer

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When a healthy embryo is created, it grows at a fairly predictable rate. The day after sperm and eggs are mixed, the embryo generally is a single cell; it then divides and grows to approximately 6-8 cells by day 3 following egg retrieval. The growth continues, and by day 4 there are dozens of cells in a ball, called a morula. By day 5, the best embryos are greater than 100 cells and have a fluid filled space within their structure; this is called a blastocyst. For some slower growing embryos, this stage is not reached until day 6 following egg retrieval.

When embryos are formed following IVF, we are faced with the decision as to when to place them back into the uterus. This was originally done on day 1, then later day 2 or 3. Recently, many programs are transferring embryos on day 5 or 6. Why the tendency to transfer later in embryo development? The answer is simple: the longer we culture embryos in the laboratory, the easier it is to distinguish which are the best of the bunch! Virtually all day 1 embryos look alike. Day 3 embryos may vary by cell number and appearance, but differences at this stage are not totally predictive of what will happen next. By contrast, a great day 5 embryo is easily distinguished from a mediocre or poor embryo.

Understanding the quality of an embryo is vitally important as it is predictive of the chances of pregnancy. To give the best chance of conception, we would like to put back the best two embryos. We can do this on day 5 or 6, and in doing so provide a very good chance for pregnancy with no chance of triplets. To have the same chance of pregnancy with day 3 embryos, we would have to transfer 3 or 4 because it is not as clear which are truly the best. Unfortunately, if they are all outstanding, there is a real risk of triplets or quadruplets! To avoid this risk, we could transfer only two embryos on day 3, but if they turn out to be mediocre then the chance of pregnancy is reduced. Thus, transferring on day 3 is trickier business than on day 5 or 6.

If this is the case, then why not always transfer on day 5/6? Some programs do just that. However, while over 80% of all embryos grow to day 3, only about 25% (1 in 4) grows to day 5. Thus, if you have relatively few embryos to start with, there is a very real chance that none will make it to day 5. If completing this process through the embryo transfer is critically important to you emotionally, such a failure to reach transfer could be devastating.

For this reason, our policy is to advise culturing embryos to day 5 or 6 whenever 8 or more embryos are formed on the day after retrieval. Our rationale is that since 1 in 4 embryos make it to day 5, you are likely to end up with at least 2 blastocysts for transfer. Conversely, if less than 8 embryos are made initially, we would suggest a day 3 transfer. Finally, if only 2 or 3 embryos are produced, a day 2 transfer is preferred by us in order to replace the embryos as soon as possible, since no selection needs to be made by laboratory personnel. After all, we assume that the uterus is at least as good an environment for the growing embryos as our incubators, and possibly even better.

However, these are merely our suggested policies. The decision will be yours. Please discuss the issues with each other as well as with your doctor. A decision should be reached the day following retrieval, when we let you know how many embryos are formed. In any event, it is best to begin the discussion now, when the level of anxiety is less. Please ask us if you would like us to facilitate or participate in this decision.