Currently we score embryos mainly from morphological observations. For oogenesis embryos, the scoring includes cell number, cell homogeneity, and cell fragmentation ratio. For blastocysts, the score includes the size of the blastocyst lumen, the denseness of the inner cell mass and the number of cells in the trophectoderm.
We have already discussed the scoring of embryos in detail, so we will not go over it again. Next, we will give examples of which embryos are transferable and which are not suitable for freezing or transfer. Take the day 3 oogenesis embryos as an example.
Both of these are beautiful 8-cell embryos with good homogeneity of the oocytes and no fragmentation, and can be rated as 3-4. Such embryos are considered by us to have a good developmental potential and can be transferred or frozen.
Both are 6-cell embryos, the first one is free of cellular debris, while the second one is more fragmented. The first 6-cell embryo can be used for freezing or transfer. The second 6-cell is not suitable for freezing or transfer because it is more fragmented and may have slightly poorer developmental potential and can continue to be cultured and then transferred or frozen at that point if it can be cultured into a blastocyst.
Both of these are 4-cell embryos, but the cell count is on the low side for day 3 embryos, and the appropriate cell count is generally considered to be 6-10 cells on day 3, with 2-6 cells being acceptable on day 2. In the absence of better options, such 4-cell embryos can be used for freezing or transfer. However, if accompanied by a higher number of fragments, with more than 50% fragmentation, they are no longer suitable for freezing or transfer.
The first is a 3-cell embryo with a small amount of fragmentation and the second is a 2-cell embryo with no fragmentation, both of which are not available for transfer because the cell count is too low and the development is too slow to have developmental potential to speak of.
This embryo had several cleavage balls visible and many small fragments with >50% fragmentation, which indicates poor cell division and almost no developmental potential, such embryos are not suitable for transfer or freezing.
Here, the requirement for third-day embryos for cell number is 6-10. Embryos smaller than 4 cells are simply discarded. Embryos larger than 10 cells can continue to be cultured if there is not much fragmentation; if there is more fragmentation they are also discarded directly, because the so-called cells seen are actually fragments.
We define fragmentation as a small vesicle less than 1/3 of the diameter of a normal ovoid. An embryo with more than 50% fragmentation will not be included as a transferable embryo and cannot be cultured further. If the fragmentation is between 20-50%, it can be considered for further culture, but rarely for transfer or freezing. If the fragmentation is less than 20% and the cell count is between 6-10, they may be considered for freezing or transfer. From these transferable/freezable embryos, the best embryos are then selected.
Why select transferable embryos for transfer or freezing? Why not freeze or continue to culture all remaining embryos?
When selecting embryos, for each patient, we would in principle pick the best 4 embryos from all her embryos and divide them into 2 tubes for transfer or freezing. But the facts are.
1. some patients have very few embryos, 1-4 in total, then depending on her specific case, at least 1-2 best embryos are frozen.
2. Some patients have very poor quality embryos and also very few in number, as mentioned before, maybe 4 cells, 5 cells, all less than 6 cells, in this case, if the embryos are less fragmented (<20%), we will select the best looking embryos for transfer or freezing, if there is a lot of fragmentation, more than 50%, then we suggest to abandon them, if in between, we suggest to continue with the embryos. Of course the opinions of patients and clinicians are also respected and sometimes such embryos are transferred or frozen.
Why are embryos with too few cells or too many fragments not suitable for transfer or freezing?
Because these embryos are destined to have no developmental potential and it would cost a lot of money to perform the transfer or freezing, which would then be very cost effective for the patient. It can be devastating for the patient to continue knowing that there is no possibility, and the hopefulness turns into disappointment.
On the other hand, embryos without developmental potential may not be revived when they are frozen and thawed. On the other hand, embryos with good developmental potential that are frozen and thawed are no different from those before freezing when they are recovered. The recovery rate of embryos in our center is as high as 98%, and the 2% of failed recoveries are considered to be mainly related to the poor quality of the embryos themselves.