The day on which an embryo transfer will occur is determined by the embryo assessment that is done early on day 3 by the embryologists. The number and quality of embryos that are growing at this time is communicated to the physician. The physician decides how many embryos are going to be transferred. If a choice as to which embryos should be transferred can be made at this time, the transfer will be performed on day 3. If there are 5 or 6 embryos that are all growing with such equal development that a decision as to which embryos should be transferred is not clear, the embryo transfer will be on day 5. This extra two days allows clear differences in growth to develop between these embryos so that a decision is more obvious.
Looking at the statistics for pregnancy results between day 3 transfers and day 5 transfers can be misleading. One approach to choosing a day5 transfer is that it allows the embryologists to gather more information about each of the embryos and then to make an informed decision as to which embryos should be transferred based on increased information. Some of the pregnancy results appear to bear this out. Day 3 pregnancy results by first HCG test and by heartbeat for all non-donor patients regardless of age are 53% and 34%. Day 5 pregnancy results by first HCG test and by heartbeat for all non-donor patients regardless of age are 75% and 64%.
However, among the patients who have day 3 transfers are patients who have only poor quality embryos. These patients are different from those patients who have a small number of very good quality embryos. Both will be transferred on day 3, but the patients with quality embryos will have a much better chance of pregnancy.
The advantage of day 3 transfer is that it allows the embryos to be placed into the uterine environment earlier, where they grow faster with a better nutritional environment than they experience in the incubator.
The likely situation is that a day 5 transfer is better for some patients (those with large numbers of quality embryos), while a day 3 transfer is better for other patients (those with few embryos to choose from).
If embryos are transferred on day 3 or on day 5, why can’t we split the difference and transfer on day 4? Wouldn’t that give us advantages of both?
On day 4, embryos are in a state of great change.. They are called morula and have approximately 40 cells. They have formed compact balls of cells if they are growing well. The compact appearance of the cells makes it difficult to assess their quality. Since there are very few visual clues at this time to differentiate quality embryos from those that are not growing well, day 4 is not a good day to transfer embryos.
If some embryos develop more cells and make more complete blastocysts on day 6, why wouldn’t it be a good idea to transfer on day 6?
The uterus has a window of time for implantation of embryos onto the uterine lining. Every day that embryos grow in the incubator, they fall a little behind the growth rate that they would experience if they were growing in the uterus. An embryo might implant if transferred on day 6, but the odds of it being ready for implantation in time would be reduced. Day 5 embryo transfer will likely produce a higher implantation rate.