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Many people have had children through fertility treatment. Since the first “test tube baby” was conceived and born in 1978, treatment options have consistently increased for those who struggle with infertility.
For procedures that include embryo transfer, embryo grading is required to give each patient the best chance at a healthy pregnancy and birth of a child. But, does it mean to grade an embryo?
What do the grades mean, and are they successful? Let’s find out together.
The process by which doctors determine if an embryo is viable, and if it is time to transfer the embryo is called grading. The process begins with an evaluation. A person’s age, medical history (including fertility treatment), and the viability and age of the embryo are all considered.
Embryo quality grading is important because it tells the embryologist which embryos are best for transfer, freezing, or testing. An embryo grading scale is used to determine if the embryo is viable, and what the success rates for a healthy pregnancy and birth are projected.
Grading is usually performed on days 3, 5, 6 or 7. Let’s look at what characteristics the doctor could be looking at for these specific developmental milestones.
On day three, the sperm and egg have united and cell division has begun. At this time embryologists can expect to see:
Many doctors now are moving away from grading on day three, because as the embryo continues to grow it may show a healthy division of cells. In other words, just because it isn’t perfect on day three, it doesn’t mean it won’t be in a few days.
On day five, the embryo has grown and divided and has begun to rapidly divide in cells and has become a blastocyst. A blastocyst is a cluster of rapidly dividing and developing cells. It is encased in a shell that, as it develops, it will begin to outgrow.
An early blastocyst day 5 embryo will be graded according to development. There are three criteria for this on the commonly used Garner Scale:
In IVF embryo grading development is graded on a 1-6 scale, 6 being the best. This scale considers:
The inner cell mass is a cluster of cells within the blastocyst. It is the cluster that will eventually form the baby. It is graded using a letter (A,B,C, and sometimes D) An “A” grade would be the best.
A: compact inner mass with many cells
B: not as compact, not as many cells
C: not complacent, fewer cells
The trophectoderm is the cells that surround the inner mass along the edges of the shell. These will form the placenta when the blastocyst hatches.
Grading the trophectoderm also employs a letter.
A: lots of cells, uniform size, no gaps, uniform size
B: average number of cells, some gaps, uniform size
C: few cells, many gaps, not uniform in size
A day 5 embryo would, ideally, be at the early blastocyst stage, and have a grading of 4 (an expanded blastocyst). This would indicate that the inner mass is present, as well as the trophectoderm, and would be developing normally.
As days 6 and 7 come around the blastocyst expands and, very quickly, outgrows its shell and hatches out. A blastocyst must hatch in order to burrow into the lining of the uterus and continue to grow into a healthy baby.
An embryo grading chart can show the grading of embryos, the success rates of pregnancy and live birth of the embryos based on the grades. When viewing an embryo grade, the number comes first (indicating the development), followed by the inner mass grade, then the trophectoderm letter grade comes last. Let’s look at some examples of embryo grading and their meanings, and their success rates.
A 3bb embryo would be around a day five full blastocyst (represented by the number 3), it would have an inner cell mass that is not as complacent with not as many cells (as represented by the first letter “b”). It would have an average number of cells, with few gaps, and the cells would be uniform in size (represented by the second letter “b”).
A 4bb embryo would have an expanded blastocyst, meaning it was growing fast, and the letters would indicate a B rating for both inner cell mass and trophectoderm.
A 5aa embryo would be at the hatching stage. It would have a very complacent inner mass with lots of cells, and would have a trophectoderm with many, uniform cells with no gaps.
A 6bb embryo would be fully hatched, have a not as compact inner mass with an average number of cells, and have an average number of trophectoderm cells that are uniform in size with less gaps.
Theoretically, the higher the grade, the higher the success rate. A study called, The Simplified SART Embryo Scoring System Is Highly Correlated to Implantation and Live Birth in Single Blastocyst Transfers, showed these statistics: “Embryos graded as good (n = 663) had a 63 % implantation and 53 % live birth rate (Fig. 1). Those graded as fair (n = 49) had an implantation and live birth rate of 47 % and 37 % respectively. There were no live births among those embryos graded as poor (n = 5).”
The grade of good could result in a healthy pregnancy and live birth. One of the issues with transferring an embryo on day three is that the grading may or may not reflect the continued success of the embryo. It is not as likely to be as accurate of an assessment, because not enough can be observed about the viability of the embryo at this time.
A study called, Day-3 vs. Day-5 Fresh Embryo Transfer concluded: “Transferring embryos at day-3 may provide the same benefits as day-5 embryo transfers to patients. However, more embryos were required to be transferred to achieve these comparable results.”
The best chance of success for a day three embryo is to transfer a higher number of day 3 embryos. However, every situation, uterus, and embryo is different. Transfer or freezing of a single embryo at day 3 could result in healthy live birth.
The transfer of an embryo either through IVF or ICSI is a big deal. It is often expensive, long awaited, and exciting. Of course, one would want to see a successful pregnancy from it.
But, how can you have the best idea of your chances of conception? That is why the grading can be so helpful. The success rates are often directly tied to the grading. The higher the grade, the higher the chance of success.
A blastocyst rating of 2bb would indicate a poor quality embryo. The stage of development, combined with the average inner mass and trophectoderm score would not have as high of a chance at survival statistically. This grade would have about a 33% chance of success.
A 3bc grading would also indicate a poor rating. As such it would have about a 33% success rate.
Embryos graded 4bb would be at about an average grade. This would give them a 50% success rate.
The score of 4bc would indicate a poor rating. These embryos do not have a high rate of success.
A 4cc embryo is also considered poor quality. This would be at a lower success rate, and likely not result in live birth.
Day 5 embryo grading can give a better understanding of grading in terms of success. At day five an embryo is usually a full blastocyst and an examination can provide details that help with the predictions.
A 5aa rating would indicate that this is a hatching blastocyst whose inner cell mass is compact with many cells. It would also show a trophectoderm that has many cells that are uniform in size, with no gaps.
This would indicate an excellent candidate for transfer or freezing. The success rate would be about 65%.
There is some disagreement among experts as to whether a 5ab or 5ba grading has higher success. You may be wondering, is 5ab or 5ba better? They are both considered good quality with success rates about 59.3% chance of pregnancy.
Any embryo with a “bb” rating is considered average. The success rates for a 6bb embryo are around 50%.
Many considerations go into the embryo grading process. Each person will be different, and each embryo will have different success rates. These grading scales and information are guidelines.
I hope that this information has given some clarity on the process, and given you hope in your fertility journey. I think it is important to note that if someone you care about is going through their own journey to parenthood, taking the time to understand the processes they may be experiencing can go a long way to helping you know how to support them.
Heitmann, R. J., Hill, M. J., Richter, K. S., DeCherney, A. H., & Widra, E. A. (2013). The simplified SART embryo scoring system is highly correlated to implantation and live birth in single blastocyst transfers. Journal of assisted reproduction and genetics, 30(4), 563–567. doi.org/10.1007/s10815-013-9932-1
Garbhini, P. G., Suardika, A., Anantasika, A., Adnyana, I. B. P., Darmayasa, I. M., Tondohusodo, N., & Sudiman, J. (2023). Day-3 vs. Day-5 fresh embryo transfer. JBRA assisted reproduction, 27(2), 163–168. doi.org/10.5935/1518-0557.20220027
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