Embryo Grading Made Easy For Embryo Donors and Embryo Recipients
Part 1 of a 3 part mini-series by Corey Burke, B.S., C.L.S. & Laboratory Supervisor and Reproductive Endocrinologist Craig R. Sweet, M.D.
Embryo grading is an important factor for both donors and recipients. Potential embryo donors want to know if we will accept their embryos for donation since part of our decision is based on the grade of their embryos. Likewise, potential embryo recipients want to know how likely the donated embryos are to survive thawing and if they are of good enough quality to build their family. Part of our estimation of success depends on the grade of the embryos.
We wish this process could be easier since there is no standardized system used in all embryology laboratories to grade embryos. Furthermore, the grading of embryos is somewhat subjective so one embryologist may grade an embryo
somewhat differently than a colleague.
Embryo grading is an imperfect process; poorly graded embryos may occasionally result in ongoing pregnancies and beautiful looking embryos may not implant and grow. Poorer graded embryos will not necessarily result in an abnormal child; they simply seem to implant and grow less frequently.
So, the appearance and grading of an embryo is an imperfect estimate of the quality of the embryo as well as the embryo’s true potential. It is, however, the best way we have to visually estimate the implantation and live birth rate of a given embryo. We will now examine one of the more common methods used to grade embryos.
When are embryos graded and cryopreserved?
Embryos are usually graded and frozen at three specific stages with “Day 0” being the day of retrieval and fertilization:
|Age of Embryos||Day 1||Day 3||Day 5-6|
|Common terminology||2PN (pronuclear stage)
2 cell stage
|Embryos are 6-10 cells||Morula &
|Grading importance||Grading not available||Grading relatively important||Grading very important|
|How often these are sent to EDI?||Rarely sent||40% of EDI embryos||60% of embryos|
Embryos cryopreserved immediately after fertilization are confirmed early on Day 1 (the 2PN (pronuclear stage), but aren’t advanced enough to be consistently graded. Freezing embryos on Day 1 is quite infrequent unless we are certain there will not be an embryo transfer. Accordingly, these embryos are rarely sent to EDI for embryo donation.
Grading Day 3 Embryos
Day 3 embryos are graded on cell number, the amount of cellular fragmentation and the symmetry of the embryo.
Most Day 3 embryos will be comprised of 6-10 cells called blastomeres. Embryos with too few blastomeres may not be healthy, so we prefer at least 7-8 at this stage. Embryos with fewer cells may not be healthy growing very slowly or may have stopped growing entirely commonly called “cell block”.
Fragments may be found in many of the embryos, which are “bits” of cells that break off from a blastomere. We prefer as little fragmentation as possible. Fragmentation is estimated as the percentage of fragmentation volume compared to the total embryo volume and is converted to a letter grade in the following way:
- 0 % = A
- 1-10% = B
- 11-25% = C
- >25% = D
A large amount of fragmentation may be caused by death of one or more of the blastomeres. The higher the fragmentation, the lower the quality of the embryo & letter grade and the less likely that the embryos will survive thawing. Embryos with high fragmentation rates implant less frequently when transferred fresh or when thawed.
Symmetry of the embryo refers to the shape of the individual blastomeres and the overall shape of the embryo. The blastomeres should all be very similar in size and generally round in shape. The scale used to grade symmetry is
- Perfect = A
- Moderately asymmetric = B
- Severely asymmetric = C
Severe blastomere asymmetry (i.e., large and small blastomeres in the same embryo) reflect nuclear/chromosomal and cytoplasmic problems suggesting the embryo is less healthy than desired. There is supporting evidence that blastomere symmetry is important and reflects overall health of the embryo. Interestingly, the overall symmetry of the embryo (round vs. oval) is of uncertain importance with some very “funny looking” embryos resulting in beautiful and healthy children.
Putting it All Together for Day 3 Embryos
For Day 3 embryos, the order of grading is the “number of cells (#c),” “fragmentation grade” and “symmetry grade”. For example:
- 7cAA = 7 cells with no significant fragmentation and perfect symmetry
- 8cBA = 8 cells with 1-10% fragmentation and perfect symmetry
- 6cBB = 6 cells with 1-10% fragmentation and moderate asymmetry.
Grading Day 5 Embryos
More advanced embryos are graded and potentially frozen on Day 5 or Day 6. These are generally described as morula or blastocysts.
Day 5 Morula Embryos
Morula embryos are difficult to grade as the cells combine, forming essentially a ball of cells that can’t really be categorized in any way other than descriptive terms:
- Morula (early)
- Compacting morula (more advanced)
While some facilities only occasionally cryopreserve Day 5 morula embryos, it is thought that the survival and implantation rates of these embryos may be slightly reduced but they are still quite reasonable, suggesting that they should not be discarded. Day 6 morulas are probably delayed in growth or may have stopped growing, may not be viable and are infrequently cryopreserved.
In order to balance the possible reduced implantation rates, it is common that more morula embryos are thawed and transferred in order to achieve success.
Day 5 Blastocyst Embryos
Day 5 blastocyst embryos are the most advanced embryos we see in IVF. These embryos are formed within 24 hour of actual implantation. Trying to grow embryos beyond this point is
technically difficult, as the embryos usually don’t survive. In addition, the window of time for implantation seemingly closes beyond Day 5 or early day 6. For example, transfer on Day 7 will rarely result in implantation. So, it simply makes more sense to transfer and/or freeze blastocysts rather than trying to grow them any further.
Along with descriptive measures, more objective grading is attempted through evaluation of the cellular expansion, the inner cell mass (which eventually becomes the fetus) and the quality of the outer cell mass called the trophectoderm (which eventually forms the membranes and placenta).
As the embryo advances in growth, a cavity called the blastocoel fills with fluid. As the cells continue to divide and the fluid collects, the embryo expands and eventually escapes its outer covering called the zona pellucida. The blastomeres continue to group together wherein the individual cell cannot be counted. As the Day 5 embryo expands, differentiates and escapes the outer zona pellucida, the grade increases numerically from 1-5.
|1||Early Blastocyst||Starting to form a fluid-filled space in the middle (Blastocoel). Grading the embryo is difficult here.|
|2||Full Blastocyst||Blastocoel forms and inner cell mass is now distinguishable. Grading can be done from this point forward.|
|3||Expanded Blastocyst||Blastocyst is starting to expand in size thinning the outer covering, the zona pellucida|
|4||Hatching Blastocyst||Blastocyst is starting to hatch out of the zona pellucida.|
|5||Hatched Blastocyst||Blastocyst is fully hatched and now ready for implantation into the uterine wall.|
Inner Cell Mass (ICM)
As the blastomeres compact to form the inner cell mass (ICM), this early fetal tissue is graded on a A-D scale:
|A||ICM with total compaction|
|B||ICM still compacting|
|D||Poor with dying cells|
The outer cells of the trophectoderm (TE) also reflect the overall health of the embryo and are graded in an A-D scale.
|A||Numerous cells forming cohesive layer|
|B||Few but healthy large cells forming a loose epithelium|
|C||Few cells present often with asymmetric distribution|
|D||Poor with degenerating/dying cells|
Putting it All Together for Day 5 Embryos
For Day 5 embryos, the order of grading is “expansion,” “inner cell mass” and “trophectoderm.” For example:
- 1 = Early blastocyst is unable to be easily graded with respect to ICM or TE as these haven’t separated well enough yet.
- 2BB = Blastocyst with partial ICM compaction with loose, large trophectoderm cells
- 3AB = Expanding blastocyst with total compaction of ICM and with loose, large trophectoderm cells
- 4AA = Hatching blastocyst with excellent ICM & trophectoderm cell layers
- 5AA = Fully hatched blastocyst with excellent ICM & trophectoderm cell layers
Day 5 embryos that are graded 4AA and 5AA are some of our favorite embryos.
Embryology laboratories strive to grow the healthiest embryos they can. Over time, they have adapted different grading techniques so the laboratories can communicate the quality of the embryos to physicians, patients and each other. Not all beautiful embryos will implant or produce a healthy child but they seem to do so more often than others. Not all poorly developing embryos will fail to implant and produce a healthy child but most do not result in live offspring.
At EDI, we try to only accept embryos that are likely to implant so embryos with less than a B rating for any category are infrequently accepted. This is done to assure our recipients that all of the donated embryos we offer are of the highest quality and provided the greatest chance for a successful pregnancy.
This is only one piece of the puzzle as many other factors influence the likelihood for success. Dr. Sweet will cover this topic in the separate blog within the next couple of months.
Also stay tuned to the upcoming blogs regarding how embryos are frozen and thawed and what techniques seem to work the best.
While embryo grading is not a perfect system, we use it to try to predict the overall quality of the embryos and their potential to survive thaw, grow and build a recipient’s family.
Corey Burke. B.S., C.L.S.
Craig R. Sweet, M.D.
Racowsky C, Vernon M, Mayer J, Ball GD, Behr B, Pomeroy KO, Wininger D, Gibbons W, Conaghan J, Stern JE. Standardization of grading embryo morphology. Fertil Steril. 2010 Aug;94(3):1152-3.