By Mo Bahrami
It is safe to assume that patients undergoing fertility treatment have one aim; for a successful pregnancy and live birth. Whilst I know that may be stating the obvious – there are many occasions where “the obvious” is not so clear. To a patient the question is very simple; at the end of a treatment cycle is there a pregnancy? The answer to that question can lead to total euphoria or absolute devastation. To embryologists, there are more questions, different parameters that are considered; that is, what are the fertilisation rates? What proportion of those embryos reached the cleavage stage of development? How many cleavage stage embryos progressed onto the morula and blastocyst stages? How many blastocysts started to hatch? How many were fully hatched. What is the proportion of cellular fragmentation? Did that embryo develop in the “normal” defined time period? These are just some of the questions that embryologists have to answer before selecting an embryo for transfer.
Suddenly things are beginning to look a little more complicated.
Some of you may be wondering how is it that embryologists select the embryo that will be transferred. Each embryo will have its development monitored through daily checks and scoring. Embryos will beobserved and a grade given based on morphology; to put it simply, what do the cells of that embryo observed and a grade given based on morphology; to put it simply, what do the cells of that embryo look like? One of the standard scoring systems used in clinics is based on the Gardner grading system. And given most clinics transfer blastocysts; the grading system presented in this blog will focus on blastocyst scoring. Before we go any further, take the time to familiarise yourself with the image below, and observe the structure of a blastocyst.
The Gardner grading system uses numbers (Grade 1 – 6) and letters (A, B & C). The numerical grades (1 to 6) are an indication of the particular stage of blastocyst development, and are as follows:
Grade1: The blastocoel (fluid-filled cavity) is less than half the volume of the blastocyst embryo. Trophoblast cells are not pressed against the zona.
Grade2: The blastocoel is more than half the volume of the blastocyst.
Grade3: The blastocyst cavity has expanded, filling the entire volume of the blastocyst, causing the trophoblast to be pressed tightly against the zona pellucida.
Grade4: Continued expansion of the blastocyst to the point where the zona pellucida appears thin.
Grade5: The embryo has begun hatching, and the cellular content appears to be bulging out of the
zona pellucida.
Grade6: The embryo is hatched – it has completely evacuated the zona pellucida.
The letter grading is given for the two main cell types of the blastocyst; the inner cell mass or ICM (which hopefully will form a healthy baby!) and the trophoblast cells (which form the placenta). When considering the Inner Cell Mass:
Grade A: Multiple, tightly packed cells. Minimal or no dead cells.
Grade B: Several cells that are loosely packed. Some cell death may be present
Grade C: Low cell numbers. Multiple dead cells present
When Considering the Trophoblast cells:
Grade A: Multiple cells forming a uniform layer. Minimal or no dead cells.
Grade B: Several cells forming a less cohesive layer. Some cell death may be present.
Grade C: Low cell numbers – cohesive layer is not evident. Multiple dead cells present.
This grading system is how we end up with a 6AA embryo, or a 3BB etc. Sometimes there is a fine line, and some embryologists will grade embryos slightly differently – for example, one scientist may give a grading of 4BC, whilst another may score that same embryo a 4AC.
In every case, it is the highest grade embryos that are selected for transfer. It is important to note that embryos are also graded prior to cryopreservation. The principle being that the better quality embryos will have a better chance of surviving the cryopreservation and warming process – and upon warming, a highly graded embryo will be transferred.
Some of you who have had exposure to this grading system may ask: “I had a 6AA embryo transferred and did not get a pregnancy – why is that? Isn’t that the best quality embryo?” In theory, yes it is – but there are many factors that will impact on that embryos ability to lead to a pregnancy – and sometimes morphology alone is not enough. I will explain why that may be the case in the second part of this blog, as well as the new craze in embryology; time-lapse imaging and whether it is a good tool. In the meantime, we wish you all the best – and remember, please consult with your fertility specialist regarding your individual needs.
Best Regards
Mo
