Answer from: Raúl Olivares, MD
There are different ways of assessing the quality of the embryos. It depends a lot on whether you transfer embryos on day-2 or day-3 or whether you transfer a blastocyst. We usually recommend a transfer on day-5. When day-2 or day-3 embryos are transferred, there are 2 different classifications. The first one is A, B, C, D – A being the best, or you can use and score system from 1 to 10, and 10 is the best. This kind of assessment takes into account things like the symmetry of the cells, whether the embryo has fragmentation if there are abnormalities inside the embryo, or how the fragmentation is distributed inside and based on that, they grade the embryo.
When we transfer a day-5 embryo, and there is a standard classification called the Gardner system, that says different things. When you see this, you first see a number, which is the grade of expansion of the blastocyst. It is not relevant in terms of success rate because an embryo can be a 1,2,3. When the embryo reaches 4, it means that it started hatching. When the embryo is 5, it means that half of the embryo has done the hatching and a 6 blastocyst means that the embryo is completely out of the zona pellucida.
What is relevant is the other two things that we assess, which is the inner cell mass, which is the part of the embryo that is going to become the embryo, and the trophectoderm, which are the cells that are going to become the placenta and the amniotic fluid later. We grade these two parts with letters A, which is the best. C is the worst. You can have different quality, for example, you can have a 3 BA or a 4 CC or a 2 AA embryo. The number is the expansion, the first letter belongs to the inner cell mass, so it’s telling us how good the embryo is. The second letter is about the trophectoderm, which is telling us how good all the other cells or the structures that are going to be the placenta are. If you have the embryo AA, the prognosis is much better than if the embryo is BC, CB, or CC.
Answer from: Harry Karpouzis, MD, MRCOG, DIUE
We have different grading systems so every laboratory may have a different grading system. As a general rule, the grading systems are tools helping us to predict which embryos have better chances of implantation. We grade differently day 3 embryos and blastocysts, day 5 embryos.
When we are talking about day 3 embryos, the grading depends on the number of cells that they have (6, 8, or 10), where the ideal number here is 8, and, also, it depends on their appearance under the microscope, whether they have fragmentation, pigmentation, etc.
When we have a blastocyst, then we usually have a grading system that has a number in front and has some letters following. The number has to do with the cavity and its size. If it is a fully expanded cavity, this is a grade 4. If it is an embryo that started hatching, this can be 5. If it is completely hatched, it might be 6. The letters, on the other hand, grade the inner mass of the blastocyst from which the final embryo is produced. The second letter grades the trophectoderm which is the part from which the placenta is created.
We measure different aspects and we look at various things under the microscope to see whether the cells are symmetrical, whether they have a grainy appearance, whether they have pigmentation, fragmentation, degeneration, and depending on that we grade the blastocysts. The best blastocyst could be a 5AA but even blastocysts that are worse grading can give pregnancy.
What is the optimal / perfect grade of embryo for transfer?
Embryo grading, how do you choose an embryo for embryo transfer? What is the best grade embryo? How are they selected?