What is a Day 5 Embryo?

Sometimes IVF seems overwhelming, but by the time a woman undergoing this treatment arrives at the point of embryo transfer, the process is actually almost complete.

Of course, there are some decisions for the doctor to make before the transfer, and much of those will depend on each individual patient and their progression through the entire IVF process. One of those decisions will be whether to do a Day 3 or a Day 5 embryo transfer.

Maybe you’ve already read a little about this as you were researching IVF but let’s make it a little clearer.

First, let’s discuss a bit of biology. An embryo begins as a single cell but then divides every 12 to 24 hours. That means after 3 days in the lab, an embryo will have somewhere between 4 and 8 cells. At this point, medical professionals usually refer to it simply as a multicell embryo.

By day 5, however, things have changed quite a bit. The embryo has an estimated 70-100 cells and two different cell types – the inner cell mass, which becomes fetal tissue, and the trophoblast cells, which lead to the placenta. This embryo is now called a blastocyst.

Day 3 vs Day 5 embryo transfer

There are a few things to consider when deciding whether to do a Day 3 or a Day 5 embryo transfer. One is that on average, embryos transferred on Day 5 have about a 50% chance of implanting, while those transferred on Day 3 have about a 30% chance.

However, this doesn’t mean that Day 3 transfers don’t work, and in fact, many women who have had Day 3 transfers go on to have successful pregnancies.

It’s also important to remember that not all embryos will make it to Day 5. In fact, about 60% of them will stop developing before then.

So, while the chance of implantation is higher with a Day 5 transfer, there are fewer embryos available for transfer.

Another thing to consider is that embryos transferred on Day 3 are usually less developed and thus have a lower risk of becoming ectopic pregnancies.

So, which is better – a day 3 or a day 5 embryo?

Well, the answer to that question is not always cut and dried. As a matter of fact, it differs with each case and infertility doctors need to make decisions that benefit the individual patient.

In short, doctors are searching for the “best” embryos for transfer. After all, they want the process to be successful. Today, the vast majority of Fertility Specialists are in favor of the Day 5 transfer because it most closely imitates natural conception, which has the embryo arriving in the uterine cavity at the blastocyst stage.

Since all embryos differ, Day 3 embryos could be considered for transfer in some instances such as when there are very few dividing embryos for consideration. However, in the vast majority of cases, including when there are a larger number of embryos involved, it is likely that many of them will look promising after 3 days. 

Therefore, waiting until day 5 to see which emerges as more optimal is a wise tactic for success. This way, the embryos selected for transfer on day 5 will have become the leading blastocysts.

In other words, the best embryos are usually those that implant on day 5. This is because they have had more time to develop and become stronger. Therefore, they are more likely to survive and result in a successful pregnancy.

While some doctors may transfer embryos on day 3, this is not as common because there is a greater risk that the embryos will not survive.

Another positive impact of waiting until day 5 is that most embryos that have chromosomal abnormalities will not survive to this point, which constitutes a sort of “self-selection” process that benefits the patient.

Furthermore, when doctors select the “best” embryo with the greatest chance of implantation, they can achieve a successful SINGLE embryo transfer which helps avoid multiple births.

While it may be tempting to choose an earlier day for transfer in order to avoid the possible risks associated with a longer culture period, it is important to remember that the success rates for Day 5 embryo transfers are significantly higher than for Day 3 transfers.

In fact, studies have shown that the implantation rate for a Day 5 blastocyst is about 50-70%, while the implantation rate for a Day 3 embryo is only about 20-40%.

One theoretical disadvantage of waiting until day 5 is that embryo transfer cancellation rates may be higher when delaying those extra few days, especially if only a few eggs were retrieved.

While this may be disappointing for the patient and explains why some doctors will opt for Day 3 transfer when eggs are limited in number, the reality is that an embryo that does not survive until day 5 in the incubator is extremely unlikely to be viable.

Another theoretical disadvantage of waiting until day 5 is that the number of embryos available for transfer may be reduced. Therefore, Day 5 blastocyst transfer strategy can help avoid the heartbreak of a negative pregnancy test following the transfer of ultimately non-viable embryos.

As in all medical practices, treating any patient should be tailored to that unique individual. At Advanced Reproductive Medicine, we offer personal care for all of our patients, designed with their individual needs in mind.

We always get to know our patients well and are eager to suggest ways that can best help them realize their dream of motherhood. For more information on our services, call us at 732-339-9300.