If your reproductive endocrinologist says you are "having a blast", (s)he is not talking about having a blast - as in having fun - but rather referring to an advanced stage of embryo development at the time of your embryo transfer. You see, an embryo may be transferred at “Day 3” when it is a cleavage-stage embryo (a.k.a. "8-cell"), or “Day 5,” when it is a blastocyst.

What Is A Blastocyst?
When an embryo reaches day three, it has 6 to 10 discreet cells (blastomeres) and will be graded for quality. The grading, done by the embryologist, assesses the number and appearance of the blastomeres. When an embryo has equal-size blastomeres with no fragmentation it can receive the highest grade (Grade I); when an embryo has fragmentation with unequal-size blastomeres, the grade is lower (Grade II or III).

In general, higher-grade day 3 embryos have a better chance of implanting successfully and developing an ongoing pregnancy. But, if the embryos are maintained in culture for a couple more days, they first form a solid ball containing approximately 30 to 50 cells, called a morula. This solid ball of cells becomes a hollow sphere with a clearly defined inner cell mass, called a blastocyst.

Many clinics keep embryos in the culture media until the fifth day to allow for improved selection of embryos to transfer. Patients who undergo an embryo transfer on day 5 or 6 after egg collection are referred to as having a blastocyst transfer.

Decisions, Decisions
Having to decide whether to wait until day 5 or transfer at day 3 may occur if you have a good number of high quality embryos. If your doctor recommends waiting to transfer to day 5, it is because your embryos may have a higher implantation rate when transferred then. But waiting does carry a small risk – the risk that no embryos will survive and you will be left with none to transfer; a profound disappointment in an IVF cycle.

You may think, perhaps that’s for the better, they were probably not viable and transferring earlier would not make any difference. After all, if an embryo fails to form a blastocysts, it probably wasn’t viable for implantation and pregnancy even if it had been transferred back into the uterus on day 3, right?

The research here doesn’t provide a crystal clear answer, but does clearly demonstrate that clinicians have observed good pregnancy rates with day-3 transfers, despite embryos sometimes being graded as having marginal quality. With the use of historical data, researchers conclude those same embryos would be within the group of those that probably would not have formed blastocysts in culture, yet implanted and resulted in pregnancy.

With the knowledge that no transfer results in no pregnancy, and even marginal embryos transferred at day 3 have resulted in pregnancy, why would anyone wait until day 5 and risk losing all the embryos?

When Is Day 5 Transfer Recommended?
Many clinics make the decision on day 3, after seeing how the embryos are doing, as to whether a day 5 blastocyst transfer will be recommended or not. Two things weigh in to reach recommending a wait until day 5 – the embryos you have are growing well, and you have a good number of embryos, a number high enough that it reduces the risk you will lose all of them before transfer. For patients that have a limited number of embryos, a day 5 transfer is less-often offered as an option. This is because the risk outweighs the benefit.

The opposite happens when a patient has a large number of good quality embryos. Which ones do you select for transfer when you have many to choose from? Waiting until day 5 often answers the question as those embryos which are not viable “declare themselves” by failing to survive, making the selection of the better embryos for transfer obvious.

While only you can make the final decision, it helps to know that waiting or not is a decision based on numbers and weighing risk; the more good quality embryos at day 3, the more chance the best ones will survive to day 5 and have a better rate of implantation. If you do not have a large number of embryos, or the embryos are graded at a lower quality, it is better to consider transfer at day 3 rather than wait since research shows that these embryos might not have survived to day 5, but still implanted and pregnancy followed. Waiting until day five also helps select the viable embryos that should be frozen and used at a later time.