- Reaching the embryo stage during IVF is a very important event.
- After the embryos are created in the IVF lab, you may be introduced to some terms: 4AA, 5AA, 3AB, 4BB, or even Grade A embryo.
- There is confusion among patients because of these letters and numbers.
The practice of embryo grading offers embryo developmental status and appearance, which is quite a powerful tool in deciding which embryos to transfer and freeze.
Still, the key is that embryo grade does not equal genetic result. A perfectly looking embryo can have chromosome abnormalities, whereas in some cases, an embryo with a less-than-perfect morphology often means a healthy pregnancy.
Embryo grading is an important step at Zemya IVF & Fertility Clinics, along with the overall clinical and embryological picture.
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What Is Embryo Grading in IVF?
Embryo grading involves identifying the best embryo through a thorough morphological assessment of specific parameters.
Embryologists will typically evaluate, based on the developing stage of the embryos:
- Cell number
- Cell size, evenness of cell sizes, or symmetry
- Fragmentation or breakdown of cells or cell mass
- Cell structure (organization)
- Stage of embryo development
- Inner cell mass (ICM)
- Trophectoderm (TE)
- Blastocyst outgrowth
Different labs employ varying grading systems. The Gardner blastocyst scoring system is one of the commonly used methods that involves a numerical expansion score combined with letter grades for the inner cell mass (ICM) and trophectoderm (TE) cells. (ASRM)
When Are Embryos Graded During IVF?
Embryos can be assessed at different stages of development.
Day 1: Fertilisation
When a woman goes through IVF or ICSI, the laboratory will determine if fertilization occurred normally. When fertilization has happened properly, an embryo will typically have two sets of nuclear material (pronuclei = 2PN) at the time when this embryo assessment is performed. (ESHRE)
Day 2 and Day 3: Cleavage-Stage Embryos
At the stage of cleaving, embryo specialists can do assessments like:
- The amount of blastomeres
- Cells’ size and symmetry
- Degree of fragmentation, if any
- How many nuclei have formed in each blastomere?
- Embryo’s overall appearance
As per the Alpha system of ESHRE, a top-quality two-day-old embryo is generally one that comprises four nicely sized cells and very limited fragmentation, and in the same way a top-quality three-day-old embryo consists of eight nicely sized cells and a very small amount of fragmentation. (ESHRE)
Day 5 or 6: Blastocyst
After some embryos develop to the blastocyst stage, which usually happens around day 5 or day 6, they keep going.
At that point, the evaluation is basically centered around:
- The amount of blastocyst opening or extension
- Inner cell mass (ICM)
- Trophectoderm (TE)
- It is there that scores such as 4AA, 5AA, 4AB, or 3BB are common.
Understanding Blastocyst Grading
Let’s break down an example such as 4AA.
A blastocyst grade has three components:
First number → Expansion stage
First letter → Inner Cell Mass (ICM)
Second letter → Trophectoderm (TE)
So:
4AA
- 4 = Expanded blastocyst
- A = High-quality inner cell mass
- A = High-quality trophectoderm
The numbers and letters describe different characteristics, so a higher number does not simply mean a “better embryo” overall.
What Do the Numbers 1–6 Mean?
In the Gardner system, the numerical score describes the degree of blastocyst expansion and hatching status.
| Grade | Meaning |
| 1 | Early blastocyst |
| 2 | Blastocoel is at least about half the embryo |
| 3 | Full blastocyst |
| 4 | Expanded blastocyst |
| 5 | Hatching blastocyst |
| 6 | Hatched blastocyst |
In short, 5AA cannot necessarily be said to be superior to a 4AA just because 5 is the larger number. The numbers refer to the stage of the embryo’s development, like expansion and hatching, and letters stand for the quality of the ICM and trophectoderm. (ASRM)
What Do A, B and C Mean?
The two letters assess different cell populations within the blastocyst.
Inner Cell Mass (ICM)
The inner cell mass eventually develops into the fetus.
In the Gardner system:
- A: Many cells, tightly packed into a prominent group
- B: Several cells, more loosely grouped
- C: Very few cells
Trophectoderm (TE)
Trophectoderm is the outermost layer of the embryo and plays a very big role in the development of the tissues that are involved in the implantation process and placenta formation.
The grading includes these factors – quantity and organisation – of the cell lines – and the level of cell cohesion:
A: A lot of cell lines forming a very close layer
B: A lower number of cells forming a less tight layer
C: Very few, bigger cells
These parameters are part of the embryo grading system proposed by ASRM and described in detail in the Gardner system. (ASRM)
What Does 4AA Embryo Mean?
A 4AA embryo is an expanded or hatching blastocyst with very good ICM and very good trophectoderm grades. It usually receives a very good score for morphology.
Still, receiving a morphological grade does not fully assure implantation or pregnancy. Embryo grading can be explained as the assessment of appearance and development.
Grading of embryos can allow embryologists to place the embryos in a category based on their overall shape with the help of morphological features.
Yet grading can not stand alone to predict the chromosome status of an embryo. (ASRM)
What Does 5AA Embryo Mean?
A 5AA embryo has reached the hatching blastocyst stage and both its inner cell mass (ICM) and trophectoderm are rated as top quality – A grades.
Here, the “5” refers to the phase of the embryo during which the cells of the trophectoderm have begun to extrude from the zona pellucida, while each A shows the excellent condition of the ICM and TE. (ASRM)
Though, 5AA does not necessarily guarantee success after embryo transfer.
What Does 3AA Embryo Mean?
A 3AA embryo is actually a complete blastocyst.
The cavity of the blastocyst that separates the ICM and trophectoderm (called the blastocoel) has expanded, and it fills up the whole embryo, while both the embryonic lineage (ICM) and the extraembryonic membranes (trophectoderm) receive an ‘A’ grade, which means they are excellent and undamaged.
As a result, a 3AA embryo can possess very good morphological quality despite its expansion number being less than an expanded or hatching blastocyst.
What Do Grades Like 4AB, 4BA and 4BB Mean?
This is where many IVF patients become confused.
Consider these examples:
4AA
4 – Expanded blastocyst
A – ICM
A – TE
4AB
4 – Expanded blastocyst
A – ICM
B – TE
4BA
4 – Expanded blastocyst
B – ICM
A – TE
4BB
4 – Expanded blastocyst
B – ICM
B – TE
The abbreviations are definitely not referring to “pregnancy success percentage”. These are simply descriptions of cell population morphology at the microscopic level.
Embryologists combine those microscopic assessments with numerous other laboratory and clinical parameters to prioritize embryo transfers. (ESHRE)
What Is a Good-Quality Embryo?
Mostly, the embryos that have better morphological characteristics are given more priority for the transfer.
For blastocysts, an embryo that:
- shows a suitable expansion,
- has a prominent and well-developed inner cell mass (ICM),
- and has a cohesive trophoblast
- that develops in the right manner,
- may get a better morphological grade.
- Still, any single embryo grade is not a promise of a pregnancy or baby.
Embryo selection is actually a clinical judgment and might be affected by other factors and morphology.
Does a Higher Embryo Grade Mean Higher Pregnancy Chances?
Better embryo quality, as judged by morphology, is proven to lead to better fertilization/embryo development or implantation rates. So, morphology is the primary embryo ranking criterion.
Still, embryo grading is not necessarily predictive of a certain pregnancy outcome.
Various conditions may affect results of IVF:
- Age of the mother
- Embryo genetic factors
- Issues related to uterus and endometrium
- Factors related to sperm
- Potential for development of embryos
- Previous reproductive success
- Health of the patient
- Laboratory and other treatment factors
At ASRM, grading based on morphology has historically been one of the main methods used by fertility clinics to select and assess embryo implantation.
Can a Lower-Grade Embryo Result in a Healthy Pregnancy?
Yes.
Embryo grading is an assessment of morphology; it’s not a guarantee of reproductive potential.
An embryo given a lower grade might still implant and result in a baby. Though, a higher-grade embryo may not be able to implant either.
To clarify, when patients think about embryo grades, they are advised not to consider it as the simplistic version: “good embryo = pregnancy” or “lower grade = no pregnancy”.
Embryo Grading vs PGT-A: Are They the Same?
Embryo grading
Evaluation of the physical appearance and development stages of an embryo.
Preimplantation Genetic testing for Aneuploidy
Determination of whether an embryo has a normal complement of chromosomes via analysis of cells obtained from the trophectoderm of the embryo.
So:
- Embryo grading reflects the morphology of the embryo.
- PGT-A reveals the chromosome copy number in the embryo.
- They provide different kinds of information. (ASRM)
In IVF, not every woman would require PGT-A, and ultimately whether it’s going to be done or not will depend on the circumstances of that patient individually and the recommendation of a physician or a clinic.
Does Embryo Grading Affect Which Embryo Is Transferred?
Actually, embryology experts are able to.
If there are several embryos to transfer, the embryologist and fertility specialist can make use of morphology to rank the embryos based on the likelihood of their success in a transfer from the highest to the lowest.
Per a survey by ESHRE, morphological scoring is the main technique that is non-invasive and widely used for the evaluation of the developmental quality of embryos as well as the ranking of embryos to be transferred.
But the ultimate decision to go for embryo transfer should take into account all the factors of a patient’s medical situation rather than just the embryo grade
What Happens to Embryos That Are Not the Highest Grade?
A relatively low-grade embryo does not automatically mean a loss of embryonic potential. An embryo may be labeled one thing at the clinic but still have enough potential to be considered in different contexts and scenarios.
The possible fates of an embryo are: to be implanted in the patient this cycle (fresh transfer).
Freezing and storage so it can be used in subsequent treatment cycles. Being tested for in vitro diagnosis (IVD) and preimplantation diagnosis (PG) as per regulations.
Further development will be kept track of. The decisions about embryos are based on the protocols and the personal treatment plan made by the laboratory team.
Does Day 5 vs Day 6 Matter?
Timing of blastocyst stage among embryos can vary. Development of some embryos to blastocysts happened already on Day 5,
Other blastocyst stages came on Day 6 or even after. In the embryo’s developmental day, there lies another aspect to the record. You cannot take it as an independent factor.
It is the combined evaluation of an embryo’s development that determines its potential for freezing or transfer.
Why Embryo Grading Is Important in IVF
Embryo grading assists the IVF team in various ways:
- They can judge embryo development through grading.
- They can differentiate embryos during one particular treatment cycle.
- Grading enables them to list potential transfers starting from the most promising.
- Based on a grading system, they can determine which embryos are good candidates for cryopreservation.
- Also, embryo grading helps the lab keep laboratory records standardized.
- It is a way to inform the laboratory and clinical team about characteristic features of various embryos.
It is quite a critical step in the embryo selection process; though, it should be remembered that embryo grading by itself is just one aspect of fertility treatment.
Final Takeaway
The process of embryo grading helps a medical professional in judging an embryo’s external characteristics and the developmental stage during IVF treatments.
Embryo grades like 3AA, 4AA, 4AB, 4BB, and 5AA appear quite difficult to understand at first, but they are really very basic as a concept.
In fact, the number represents the stage of blastocyst expansion and hatching, whereas the 2 letters refer to the inner cell mass and trophectoderm, respectively.
The grade may look quite good, but none of the grades really assure a successful implantation, pregnancy, or the birth of a live child. Because of this, we interpret embryo grading along with other factors like age of the patient, prior reproductive history, clinical factors and, where suitable, additional embryo testing.
Embryo assessment done via microscopic inspection is a crucial component of IVF treatment that supports the fertility team at Zemya IVF & Fertility Clinics, Delhi, in making decisions for the embryo transfers and cryopreservation procedures.
