Several genetic factors help determine if an embryo is suitable for embryo transfer. However, genetic testing is required to analyze such factors.
Normally, embryologists don’t go that way. Instead, they use an IVF embryo grading chart after analyzing the embryo’s appearance.
An embryo grading chart helps embryologists select the best one to boost the pregnancy success rate.
Knowing the concept of embryo grading and success rates and how to read the embryo grading chart will help parents know what to expect in the fertility journey.
Therefore, our fertility lawyer in Canada has compiled everything you need to understand the embryo grading chart.
Read More: IVF Cost in Canada Breakdown: How much is IVF in Canada?
What is Embryo Grading?
Couples receive images of their embryos during an IVF round, accompanied by a grading score that may seem quite hard to comprehend for most.
Experts utilize a method called embryo grading to evaluate the quality of IVF-produced embryos before their placement into the uterus.
Under a microscope, the embryos are examined, and grades are given according to specific criteria, including the number of cells, cell division, and the degree of fragmentation.
An embryo’s grade summarizes its appearance, aiding in clinical and laboratory judgments on which embryo to choose for transfer, when to perform the transfer, and whether to consider transferring two embryos.
Embryo grading is combined with the patient’s age, past IVF cycle results, medical history, and the number of embryos available in each cycle for experts to reach a conclusion.
Additionally, embryologists frequently use an embryo’s grade to determine whether it is suitable enough to undergo procedures like biopsy (for pre-implantation genetic testing or PGT), cryopreservation (freezing), and thawing.
For instance, the genetic activity occurring within the embryo’s cells aids in explaining why some embryos with a seemingly high grade may not result in a successful pregnancy.
Characteristics of an Embryo
Before we start decoding the embryo grading chart, it is crucial to know the terminologies related to an embryo:
1. Morphology
It is a basic term that refers to how the embryo appears physically.
2. Zygote
A zygote is formed when a sperm fertilizes an oocyte (egg).
3. Compaction
Compaction occurs when cells (blastomeres or embryonic cells) change their shape and tightly align against each other to form a compact ball.
This stage typically occurs late in day three to day four of analyzing the sample.
4. Morula
The embryonic cells will condense to form a morula following several rounds of cell division in the embryo. A morula is a group of 12 to 32 cells that resembles a blackberry.
This usually happens four days after fertilization when the cell enters the stage of developing the blastocyst.
5. Formation of Blastocyst
It is the developmental stage that is normally called blast. It happens on days five and six of culture. A morula transforms into a blastocyst when it develops a fluid-filled space called the blastocoel cavity.
The fluid separates the cells in the blastocyst into two layers: trophectoderm (outer cell mass) and inner cell mass.
6. Inner Cell Mass
Simply put, it is the collection of cells on the inside of the blastocyst that gives rise to the actual fetus.
The cells that envelop the blastocyst’s exterior are known as trophectoderm. They contribute to implantation and eventually develop into the placenta.
The Embryo’s Life Cycle
Now that you know the important features of an embryo, let’s move to the life cycle of an embryo.
This will help in the deeper understanding of the embryo grading chart. A woman’s ovaries are stimulated with medication during an IVF cycle to produce many follicles or eggs.
These follicles mature into oocytes (mature eggs) that are placed in a Petri dish containing a special growth medium and capacitated sperms. This is the conventional method of IVF.
Another method of IVF is intracytoplasmic sperm injection (ICSI). During ICSI, a sperm is injected directly into a mature oocyte to fertilize it. This process is mainly for couples with unsuccessful conventional IVF results.
ICSI guarantees a high rate of fertilization and complete control over the process. However, this option is more expensive than conventional IVF.
Fertilized eggs are placed in an incubator to enable the development of embryos. Embryologists closely monitor each embryo as it develops throughout time after fertilization.
Formation Of an Embryo
Day One: A new single cell, known as a zygote, forms during fertilization when an egg and sperm unite.
Day Two: The embryos are now at the cleavage stage of development, where they will split and produce two separate cells. The cells in the embryo will now divide and grow every day.
Day Three: This is the phase where it becomes possible to observe the embryo within the cell. The embryos continue to divide. During this phase, the embryo usually contains 4–8 cells.
Day Four: The embryos keep dividing quickly. They usually have 12–32 cells and phase into the morula stage. If they reach this point, the cells will probably advance to the blastocyst stage the following day, and the number of cells significantly increases.
Day Five: Embryos should be developing into blastocysts by the fifth day. The embryo’s area inside the zona pellucida, or protective shell, becomes insufficient for the growing number of cells inside the embryo.
As a result, this protective layer surrounding the embryo breaks.
Embryo Grading Chart: How To Interpret the Grading System
Embryos can proceed with their development following the initial grade. The quality of embryos can improve after the initial grade.
For example, an ICM that was first graded as C may continue to develop and improve to an A grade in a day.
So, the use of an embryo grading chart helps differentiate between bad, average, and high-quality embryos. But, this embryo grading chart may not be the same for every fertility clinic.
Therefore, you may see a disparity in the embryo grading chart values if you try out various clinics for different IVF cycles.
Considering that, this guide explains the widely used system for grading embryos–the Gardner grading system. This system has three categories:
Inner Cell Mass Score (ICM): start from A to C (it is a quality grade with A representing the best quality)
Blastocyst formation stage: Starts from 1 to 6 (it represents developmental stages with 5 indicating the most developed phase)
Trophectoderm score (TE): start from A to C (it is also a quality grade with A being the best quality)
Degree of Expansion
This is another terminology that you need to know when reading the embryo grade chart.
The size of the blastocyst cavity is indicated by the degree of expansion, which is a value between one and six. On Day 5 or Day 6 of embryo culture, or 5–6 days following fertilization, this cavity begins to form and becomes readily apparent.
Here is how you can determine the embryo’s quality with the help of a chart:
| Blastocyst Stage | Quality Grade | Processes that Take Place |
| Early blastocyst | 1 | The blastocyst cavity is less than half of the entire embryo’s volume |
| Blastocyst | 2 | The blastocyst cavity covers half or more of the embryo’s mass. |
| Full blastocyst | 3 | The blastocyst cavity takes over the embryo |
| Expanded blastocyst | 4 | The surrounding membrane (zona pellucida) starts to get thin, and the blastocyst cavity volume becomes greater than that of the early embryo. |
| Hatching in Process | 5 | The outer cells start pushing the membrane covering them. |
| Hatched blastocyst | 6 | There is no contact left between the membrane and the blastocyst. The blastocyst has hatched out of the membrane. |
| Structure of the Blastocyst | Grade | Explanation |
| Inner Cell Mass | A | Most of the cells are closely joined |
| Inner Cell Mass | B | Several cells show a loosely packed arrangement |
| Inner Cell Mass | C | Few cells |
| Blastocyst Structure | Grade | Explanation |
| Trophectoderm | A | Numerous cells form a tightly woven epithelial lining |
| Trophectoderm | B | Reduction in the number of cells |
| Trophectoderm | C | Very low number of cells forming a loose epithelial lining |
Embryo Grading and Success Rates
It’s difficult to predict the success rate based solely on the IVF embryo grading chart. Sometimes, the results may be opposite from what the embryo grade chart translates.
| Poor Quality | Good Quality | High-Quality | Discard |
| 2BB, 2AB, 2AA | 3AA | 1AA, 1AB, 1BB, 1BC, 1CC | |
| 3BB, 3BC | 3AA, 3AB | 4AA, 4AB, 4BA | 2BC, 2CC |
| 4AC, 4BC | 4BB | 5AA | 3CC |
| 5BC | 5AB, 5BB | 4CC | |
| 6BC | 6AA, 6AB | 5CC | |
| 6CC |
The best embryo for transfer is 5AA, which exhibits the greatest quality rating for both the developmental stage and the cellular makeup.
The chances of 5AB and 5BA embryos surviving an embryo transfer are also rather high.
The likelihood of a successful pregnancy following an embryo transfer decreases with poor quality grade.
On the fifth day, the embryo should ideally have a quality rating of 3, 4, or 5.
From these three grading numbers, 4 and 5 denote the best development. Trophectoderm grades of A and B and inner cell mass are the two key markers of embryo survival following transfer.
Clinical studies on this subject have shown that blastocysts rated 3AA, 4AB, 4BA, or higher that are used for transfer have a clinical pregnancy rate that is, on average, somewhat greater than 60%.
Pregnancy rates for embryos that have 4BB, 4AC, 3BB, 2AB, and 2BA marking on the embryo grading chart have been estimated to be between 45 and 50%.
The lowest pregnancy rate is about 30% for blastocysts with grades of 3BC, 4CB, 4CC, and 2BB in the embryo grading chart.
In short, following the transfer of a blastocyst with an A-grade or B-grade TE and an A-grade ICM, pregnancy rates increased.
Transferring a single embryo may have a lower success rate than transferring several embryos.
Alternatively, multiple embryo transfers may end up in a successful pregnancy.
Another study published in the US National Library of Medicine states that higher-quality embryos have greater clinical pregnancy rates than lesser-grade embryos.
Remember that your overall chances of success with IVF are also influenced by other factors, including the embryo’s genetic composition, the laboratory conditions at your clinic, and the experience level of the embryologists.
FAQs
What does an embryo grading chart include?
Cell number, symmetry, and fragmentation are some of the criteria commonly used to grade embryos. Grades ranging from A to C or numerical scores are assigned to them; higher grades denote higher quality.
Is a higher pregnancy success rate associated with a higher embryo grade?
Although higher-grade embryos have a better chance of pregnancy, other important factors that affect IVF success include the mother’s age and overall health.
Is it possible to have a healthy pregnancy with low-quality embryos?
Yes, healthy pregnancies can still result from lower-grade embryos. Although the embryo grading chart offers direction, it may not ensure success or failure.
Does every clinic employ the same system for grading embryos?
No, different clinics may have different grading patterns. Some employ letter grades (A, B, and C), while others may employ specific descriptive phrases or numerical systems.
Can embryos with a good grade have defective chromosomes?
Even with good grades, embryos can have chromosomal abnormalities.
Contact Our Fertility Lawyer In Scarborough to Understand More About Embryo Freezing or Transfers
The embryo grading chart helps the fertility clinic determine if they want to freeze, transfer, or get rid of the embryo.
This does not imply that other embryos that may have been given a lesser grade won’t be successful if the transfer of the high-grade embryo does not result in a pregnancy.
Every embryo that satisfies the requirements for freezing or transfer has the potential to develop into a healthy baby.
Hence, it’s crucial to remember that embryos of any grade and at any stage of development can result in pregnancy.
If you want information on fertility & adoption laws, contact our fertility lawyer in Whitby.
Author Profile
- Athena Narsingh Barnett is a trusted and knowledgeable lawyer in Scarborough. Her expertise spans real estate law, family law, adoptions and fertility law. A lawyer by profession and a humanitarian by heart, Athena Narsingh Barnett wants to help people become more familiar with the legal system and be well-informed to make important legal decisions.
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