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Understanding the Embryo Grading System




What Is Embryo Grading System?


Embryo grading is something that every couple will come across during the IVF treatment cycle. The embryo grading system helps fertility doctors and embryologists to determine:

  • Which embryo to transfer
  • When to transfer
  • How many embryos to transfer

Embryo grading is done in a laboratory by an embryologist. The grading process mainly focuses on the appearance of the growing embryo(s). This will help the embryologists to select the embryos which have the best chance of implantation.

However, this grading system is not the only parameter used to predict live birth. It will be used along with other important information such as the patient's age, fertility history and other relevant health conditions that may affect the chance of implantation. It is important to select the best embryo in order to increase the chance of implantation, and reduce the risk of multiple pregnancies.



How Does Embryo Grading System Work?


Typically, the embryologists will monitor the development of the embryos and prepare the embryo grading reports on Day-3 and Day-5 (the counting starts as soon as fertilisation of the sperm and the egg takes place).



Day-3 Embryo Grading


After 72 hours, the embryo will be at the stage in which it is dividing itself without changes in size. By using a high-power microscope, the embryo will be examined and graded based on the following criteria.

  1. The number of cells in the embryo

    The embryo will have a count of 6, 8, and 10 cells with an outer layer at this stage. Nevertheless, not all cells divide at the same time, hence there can also be variations of 3, 5 and 6 cells as an outcome.

    Some might argue that 8 is the ideal number of cells, and any more than that indicates that the embryo is growing too fast and could burn out the energy provided by the egg before having the chance to proceed with the implantation stage. However, others may otherwise argue that an embryo with 8 or more cells has a better chance of live birth. 

  2. The shape or appearance of the cells

    Next is to grade the embryo on how it looks. Your embryologist will be looking for same-sized cells, and none or less fragmentation. Fragmentation is when a portion of the cells breaks off and forms something called a bleb. Bleb does not contain a nucleus, therefore it is not considered a cell and won’t be counted as such. 

As Day-3 embryo grading is rather subjective, different fertility healthcare may have different sets of grades in place for their practices. For example, some may classify the embryo as Grade A, B and C, and others may classify it as Grade 1 to 5. All grades are applicable as long as the criteria remain. The following table is a sample of how the grading can be presented:


Embryo Grades Description of the Embryo Criteria
High Grade Cells are same-sized, with none or minor fragmentation
Medium Grade There are more irregular-sized cells than same-sized cells, with minor or moderate fragmentation
Low Grade Cells are irregularly sized, with major fragmentation



Day-5 Embryo Grading


Upon Day-5, the embryos are known as blastocysts. They are still continuing to divide and the number if cells are increasing. On top of that, the blastocysts are now growing in cell size and starting to differentiate into specific cell types. Some cells will form into inner mass cells (ICM) which eventually become the fetus. Meanwhile, some cells will form into trophectoderm epithelium (TE), which develops into a placenta and other tissues needed for pregnancy. Blastocysts will now start to outgrow the spaces within the outer layer of the embryos. The outer layer of the embryos will now thin down and eventually burst in preparation for implantation in the uterus lining.

Blastocyst grading is more complicated. It comes with a more standard grading as more fertility centres are using Gardner and Schoolcraft’s three-part scoring system. The blastocyst will be graded based on the following criteria:

  1. The degree of expansion of the blastocysts (also known as blastocoel cavity)

    The blastocoel cavity is a fluid-filled cavity that forms within the embryo. The degree of expansion of the blastocoel cavity will be measured and graded based on a numbering scale of Grades 1 to 6, with 6 being the most expanded.

  2. The quality of inner cell mass (ICM).

    The ICM is graded for the compaction of cells. Compaction of cells refers to the process of cells increasing their cohesion to band more tightly together. ICM is graded by either Grades A, B or C, with A being the best quality. 

  3. The quality of the trophectoderm epithelium (TE).

The quality of the TE is measured by the number of cells and graded based on Grades A, B and C, with A being the best quality.


Each of these criteria will be combined to create the embryo grade (numbers, letters, letters). For example, if you have a Day-5 blastocyst with grade 5AA, this can be read as

  • 5 - the blastocyst is a hatching blastocyst
  • A - the blastocyst has the highest quality of ICM
  • A - The blastocyst has the highest quality of TE cells

The chart below shows all the possible grades, as well as what each grade means.


Number: Blastocyst Stages of Expansion


Grades of Expansion   Names   Meaning
1 (Worst)   Early blastocyst   Blastocoel cavity fills less than half of the volume of the embryo
2   Blastocyst   Blastocoel cavity fills more than half the volume of the embryo
3   Blastocyst   Blastocoel cavity fills the embryo
4   Expanding   Blastocoel cavity is greater than embryo and the outer layer is thinning down
5   Hatching   TE starting to burst through the outer layer 
6 (Best)   Completely hatched   TE has burst through the outer layer 


First Letter: Grade of ICM (which develops into a fetus)


Grades of ICM   Description
A (Best)   A cohesive layer of many cells – meaning there are many cells and the cells are tightly packed.
B   Loosely packed cells
C (Worst)   Few large cells or very few cells


Second Letter: Grade of TE (which develops into the placenta and other supporting cells)


Grades of ICM   Description
A (Best)   Large numbers of tightly packed cells
B   Loosely layer of cells
C (Worst)   Few numbers of cells



Embryo Quality and IVF Success Rates


The embryo grading system is very complex and subjective. The grades given to the embryos are not set in stone as the embryo is an ever-growing lump of cells. A lower-grade embryo does not mean it will stay that way forever, it is common for a lower grade embryo to become a higher grade embryo overnight. The takeaway here is not to be discouraged or hung up on the grade of the embryos alone in determining the success of an IVF cycle. 

Always remember that the embryo grading system is just a tool that embryologists use alongside other information such as the patient’s age, health condition and medical history. It is not the only parameter used to predict live birth. The decision regarding which embryo to transfer, when to transfer and how many embryos to transfer can only be made after considering all factors that can contribute to a higher success rates for an IVF treatment.


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