Embryo grading and quality assessment explained
At the beginning, there was an egg… In this promising way, Małgorzata Wójt and Agata Olborska from the Embryology Lab at Salve Medica begin their presentation on embryo grading and quality assessment. They go onto help us understand all the complicated terms and the results from the area of embryology that are so crucial in the success of IVF procedures.
Oocyte quality importance
The egg (or oocyte in the medical nomenclature) is the most important factor in embryonic development. It may be that the good looking oocyte does not always turn into a good blastocyst. There are different reasons for this which could involve the quality of semen and genetic competence of the embryo itself. However, Małgorzata Wójt admits, the better the oocyte, the higher the blastocyst quality. When it comes to the abnormal oocyte, it almost always results in a poor embryo and poor development to the stage of a blastocyst.
Ferilisation – day 1st
On the first day after correct fertilisation or ICSI, we observe that the oocyte has two polar bodies and two pronuclei. For embryologists the most important is the nucleoid – a small structure inside the nucleoli. Its structure and distribution inside the nuclei is crucial for embryo development. According to Małgorzata Wójt, the first day is important as afterwards it is not possible to assess fertilisation abnormalities. And abnormal fertilisation gives genetic disturbance. Genetically disturbed embryos can develop to healthy looking blastocysts but they will not give birth to a healthy baby.
In order to support their work, embryologists use a time-lapse system. It’s a modern incubator with a built-in camera and software to collect a lot of data during the embryo culturing. Time-lapse embryo monitoring enables continuous and non-invasive embryo observation without the need to remove the embryo from the optimal culturing conditions. It gives embryologists more information. For example, it allows for more accurate morphological assessment and helps in identifying embryos with a higher implantation potential.
Embryo day 3
Then Agata Olborska takes us to day 3 when we encounter a cleavage stage embryo. Quality assessment of day 3 embryos is important to select the top-quality embryo for the transfer. The assessment is based on cell number and cell morphology. It’s important for a patient to know that different laboratories use different assessment standards.
You may be interested in reading more about: Day 3 vs. day 5 embryo transfer success rates
Embryo scoring system
The most common scoring assessment for the blastocyst stage is developed by Gardner and Schoolcraft. It is based on two main criteria: the expansion state of the blastocyst and the consistency of the inner cell mass and the trophectoderm cells. The grading scale in the blastocyst stage embryo assessment has 3 degrees, from A which is the best to C – meaning the worst. We can also distinguish 4 main expansion states of the blastocyst: the early blastocyst (1), the blastocyst (2), the full blastocyst (3) and the expanded blastocyst with the thin zona pellucida (4). If the blastocyst begins to hatch – free itself out of the zona pellucida – we can describe two more grades: the hatching blastocyst (5) and the hatched blastocyst (6).
Another way of assessing embryos is sequential assessment. After 18-19 hours following ICSI, embryologists assess the presence and quality of pronuclei. The embryo that is 25-26 hours after ICSI should be cleaved into two cells. After 42-44 hours, there should be at least 4 blastomers visible in the embryo while 66-68 hours after ICSI, this number should go up till 8. When 90-92 hours have passed since ICSI was performed, the embryo should be at the morula stage. After 106-108 hours – it reaches the blastocyst stage.
Agata Olborska emphasises that an embryo assessment provided by an embryologist is key to choosing the best embryo to transfer. However, embryologists should refrain themselves from crossing out the chance for the blastocyst stage due to the less than perfect embryo development.
Embryo day 5
Finally, Małgorzata Wójt asks the most significant question that all embryologists ask – is it better to transfer embryos on the cleavage stage or on the blastocyst one? The answer seems to be simple: performing blastocyst transfer is more beneficial as the morphologic assessment of day 3 embryo has limited predictive value for subsequent embryonic development. Maintaining the embryo culture until day 5 allows for the correct identification of the best quality embryos. This, in turn, results in the highest probability of implantation success.
You may be interested in reading: Embryo development in IVF Lab – a complete guide written by an embryologist
The presentation and answers to all the questions from attendees were given by experts:
- Agata Olborska – Junior embryologist at Salve Medica and working in the clinic for more than two years. Graduated from the Division of Medical Analytics at the Faculty of Pharmacy at the Medical University of Łódź. The member of ESHRE.
- Małgorzata Wójt – Head of the embryological laboratory at Salve Medica with 18 years of experience in the field of fertility.
She specializes in assisted reproduction procedures. She completed a series of training and workshops in Poland and across the world including the ESHRE certificate of Senior Clinical Embryologist, which entitles the clinical embryologist to practice in all European countries.
Questions and Answers from the event
Is there an expected % of day 3 that will make it to day 5?
Generally, half of day 3 embryos will develop to the blastocyst stage. It’s normal development.
I have embryos graded 5BB. What does it exactly mean and what could be my success rate with that gradation?
It is a hatching blastocyst. It may not be the top quality, but if it’s a day 5 embryo, then it’s a quite good one. But the success rate does not only depend on the blastocyst quality. It also depends on a patient’s endometrium and their age.
I have had a 6BB transfer today.The doctor said it is a very good quality embryo. Is that correct?
Maybe it’s not a very good quality embryo, but still a good one. Grade 6 means it’s hatched from the zona pellucida.
What is the maximum grade for an embryo?
Grade 6 is a completely hatched blastocyst. When we can assess the inner cell mass and the trophectoderm, AA is the top quality.
The clinic, where I had 3 transfers, has also blastocysts on the list (embryo donation). The blastocyst highest expansion grade is 6, for example 6BB. Is it ok to transfer such an embryo?
Generally, I don’t like grade 6 expansion because it’s a very sensitive embryo. It doesn’t have zona pellucida which makes it safer. Sometimes it doesn’t depend on the laboratory because the blastocyst hatch is so fast. But generally, we try to avoid such a situation.
How about frozen embryos? Is there another assessment done just before the transfer?
No, the assessment is the same. Now many clinics are using vitrification for the frozen embryo. Vitrification is a very safe way of embryo preservation as it doesn’t change the quality of the embryo. But you should remember that if we thaw an embryo e.g. 5 or 6 hours before the transfer, the blastocyst could grow and expand so the assessment could change. For example, when we vitrificate a blastocyst at stage 2AA and we leave it in the incubator for 6 hours, it could be at 3AA stage when we transfer it. And that’s normal.
If you had 1 low-quality blastocyst and 1 of a higher quality, would you transfer both?
No, I wouldn’t because every embryo works differently with endometrium. Of course, when we have two similar embryos, it is possible to transfer both. But not in this situation. Transferring one embryo of low quality with the other of higher quality is not a good solution.
Is a completely hatched embryo not the best for immediate implantation?
It’s a very difficult question. Of course, it’s completely ready for implantation but it’s not a very safe situation. You should remember that when the embryo is totally hatched, it’s very sensitive. And then the transfer should also be provided really carefully.
Can you give us any tips on what you can do to help implantation after embryo transfer?
Firstly, in our clinic, we use Embryo Glue very often. It is a special hyaluronan-rich media that helps in embryo implantation. Secondly, we perform laser-assisted hatching.
Dermatology and radiology already enjoys support by Artificial Intelligence (AI). What is your opinion about embryologists supported by already available AI?
It’s a difficult question. We’re not convinced about AI in the embryology lab. It’s still a human factor that’s decisive in the lab.
Can you freeze embryo blast at any quality or does it have to be high? Also, what is the rate of thawing embryos?
In our clinic, we freeze embryos at the blastocyst stage and of any quality. We once had a couple with only one blastocyst assessed as 2CC. So it was really poor quality but they decided to transfer it. We conducted assisted hatching and this blastocyst was implanted successfully despite its poor quality. So we freeze embryos of all quality. The rate of thawing embryos is almost 100% because the embryo is always very safe during vitrification.
I read that in mosaic aneuploidy, the embryo can sometimes segregate and destroy the abnormal cells and end up developing into a healthy baby. Was it the situation in the earlier video with 3 embryos that you showed us?
Mosaicism is a different situation than aneuploidy. Aneuploidy is a situation when we have, for example, 3 sets of chromosomes. Mosaic aneuploidy is when we have only one or more chromosomes. It’s different from what we have seen earlier in the video. But the fact that some cells were not a part of compaction doesn’t mean that this embryo is a mosaic aneuploidy. It’s not equal.
Okay, so can it segregate bad cells for some trisomy or monosomy chromosomes then?
Trisomy is a disease, for example, Down syndrome, monosomy generally is lethal. Sometimes we have a mosaic embryo. The genetic quality of this blastocyst depends on how many cells are segregated and what is the level of mosaicism in this embryo. If it is a low level of mosaicism, sometimes this embryo is ready to transfer. But if we have an embryo with a whole trisomy or monosomy, then this embryo isn’t transferred.
I have 2 straws of embryos in the freezer. One with 3AA and 3C, the other straw with 3BB and 3BA. Which straw should I use first?
Of course, it also depends on the day it was frozen. If there are only 2 straws, firstly we use the one with 3BB and 3BA. It may not be the top quality but it’s still a good one. Whilst 3C is a poor quality embryo.
What is your opinion of PGT-A testing damaging the embryo? I am hoping to be able to use Non-invasive chromosome screening (NICS) instead in order to test the embryo for mosaicism without damaging it.
I think when PGT-A aid testing is performed by an experienced embryologist it’s quite safe. If you have a blastocyst which is built of approximately 100-120 cells and you take only a few for testing, it’s still safe to the embryo.
Some embryos are still frozen by the old slow freezing method. What chances of survival do such embryos have after thawing?
Generally, the slow freezing method is used in the blastocyst stage. It’s different in the earlier stages. But in the blastocyst, chances of surviving after throwing are about 90%. In the cleavage embryo it’s about 85-90%.
Can you transfer a slow growing embryo on day 7?
Not in a fresh transfer because then we transfer only on day 5. But in my career, I transferred frozen day 7 embryos only three times and two of these transfers were successful. It’s a completely abnormal situation for me but sometimes it’s okay. I read some paper where scientists compare the pregnancy rate of different embryo transfers. The first was the transfer of frozen embryos that achieved the blastocyst stage on day 5 or 6. The second group included embryos which achieved the blastocyst stage on day 7 or day 8. The pregnancy rate in the first group was approximately 38% and in the second group – only about 5 to 6 %. So it’s a huge difference. So usually we avoid such a situation and in fresh transfer, we transfer only on day 5.
What could I ask the clinic to know if the embryologist is experienced and skilled enough to do a good biopsy for PGT-A testing without damaging the embryo? How many cells can safely be taken? I know some labs do 2, some do 5.
I think that during the embryo biopsy we have the best results when we take 5 to 8 cells. Of course, we’re talking about the blastocyst and not about a day 3 embryo. 5 to 8 cells is enough to conduct good quality testing.
Do you think that labs which are not using time-lapse to observe the embryo development are at a disadvantage in judging the embryo quality?
I don’t think so. The time-lapse is a useful device and a very important tool for embryologists but it has no influence on embryo quality.
Does time-lapse not help to observe pronuclei count and compaction?
Of course it helps. But if a laboratory doesn’t have time-lapse, it can organise its work differently. At times when we didn’t have time-lapse system, we conducted the evaluation on the basis of different time methods. So time-lapse is a very useful tool but it doesn’t change the embryo quality. If an IVF lab doesn’t have a time-lapse incubator, they organise they work in the best possible way to conduct the proper assessment. So, for example, there is always somebody in the lab to perform the assessment after 18 or 19 hours after microinjection.
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