Understanding the different embryo grading scales is crucial in knowing how your doctor determines which embryos are best for the embryo transfer.
Grading scales aren’t the only factor in determining whether an embryo will develop into a pregnancy, but seeing what goes into the IVF embryo grading system will allow you to feel more confident on this journey.
When it comes to preparing for egg retrieval or any other medical questions you may have about your IVF journey, we can connect you with a list of reputable fertility clinics near you. Contact us online to learn more about the support we offer in addition to helping you find a fertility clinic.
Whether that support looks like getting help to find the perfect surrogate or completing your surrogate planning questionnaire while your embryos are being created, we’re here for you.
Embryo Grading Scale
The IVF embryo grading system helps your medical professionals decide:
Which embryos are ready for transfer When to transfer the embryos How many of the embryos to transfer
This grading system doesn’t guarantee success, there are many more factors that lead to a successful pregnancy but the embryo grading scale is a tool to help make that decision.
Embryo grading is done at two points during the developmental stages, the cleavage stage on day three and/or the blastocyst stage on day five.
Embryo grading assigns good, fair or poor assessments of the embryos to decide which are best to transfer but depending on when the grading is done, different characteristics are evaluated.
Day 3: Embryo Grading Scale
On day three, or the cleavage stage, embryo grading looks at:
- Number of cells
- Appearance of cells
- Degree of fragmentation
The grading scale can vary for day three embryos, depending on the clinic you work with, but most use numbers (1-4) or letters (A-D) with the 1 or A being the best grade. Here’s an example of what the embryo grading scale could look like:
Embryo Grade Description 1 or A Cells are similar with no fragmentation 2 or B Cells are similar with minor fragmentation 3 or C Cells are mostly similar sized with moderate fragmentation 4 or D Cells are either similar or not with moderate to heavy fragmentation
Day 5: Embryo Grading Scale
Day five embryos continue to divide while the different cells increase in number and size. For embryos that reach the blastocyst stage on day five, there’s a different IVF embryo grading system than for day three embryos.
On day five, or the blastocyst stage, there are two cell types:
- Inner Cell Mass (ICM): A ball of cells that will eventually grow into the fetus.
- Trophectoderm Epithelium (TE): A sheet of cells that will form the tissues needed during the pregnancy like the placenta.
A blastocyst is a cluster of dividing cells made by a fertilized egg aka the early stage of an embryo.
The ICM and TE are crucial for a healthy pregnancy and graded to categorize how much the embryo has expanded to this point.
The most commonly used embryo grading scale for day five embryos is the Gardner system.
The higher the number indicates what stage the blastocyst is currently in, in terms of expansion and hatching status, with 1 being an early blastocyst that has a blastocoel cavity that’s less than half the volume of the embryo and 6 being a completely hatched blastocyst with the TE bursting through the zona pellucida. The higher the letter grade, the better the ICM’s quality. The letter grade of A indicates a cohesive layer of many cells while a letter grade of C means few large cells. Similar to the ICM quality, the higher the letter grade, the better the TE’s quality. Receiving the grade of the letter A would mean there are many tightly packed cells while a letter grade of C means that there are few cells. Gardner Blastocyst Grading System
Below are some examples of the types of grades that your embryos could receive and what they mean under the Gardner system:
Embryo Grading 1ab
The number one would indicate that it’s the earliest stage for the blastocyst with a blastocoel, a fluid-filled cavity that forms in the embryo as it grows, that is less than half of the volume of an embryo with few cells forming a loose tissue that covers internal and external surfaces on the body.
This tissue, also called the epithelium, not only forms the covering of all internal and external surfaces of the body, it lines body cavities and hollow organs while being a major tissue in the glands.
Embryo Grading 3
A grade of the three would mean that it’s a full blastocyst with a blastocoel that completely fills the embryo.
Blastocysts with a grade number of three or higher are considered full blastocysts.
Other examples of the embryo grading scale for embryos with a grade of three include:
- Embryo Grading 3aa: An embryo with the grade of 3aa is a full blastocyst with a blastocoel that completely fills the embryo. Many cells are tightly packed together, forming a cohesive epithelium.
- Embryo Grading 3ab: The sole difference between the embryo grading of 3ab to 3aa is that the 3ab graded embryo TE has few cells that form a loose epithelium.
Embryo Grading 4aa
If an embryo is graded as 4aa, it’s an expanded blastocyst with a blastocoel volume that is bigger than one at an early developmental stage with a thinning, hard outer layer of cells that’s also known as the zona pellucida.
It’s tightly packed with many cells that form a cohesive epithelium.
For embryos that are graded 4ab, the lone difference is that instead of a tightly packed layer of skin cells there are fewer cells that form a loose layer of skin cells.
Embryos graded 4bb see several cells loosely grouped that also form a loose layer of skin cells.
Embryo Grading 5aa
If the IVF grading system grades an embryo 5aa, the blastocyst is hatching with the TE starting to break through the zona pellucida. These skin cells are tightly grouped together.
Embryo Grading 6bb
An embryo at this stage is completely hatched with a loose layer of skin cells.
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Embryo Grading Scale FAQ
Anything about 3aa can be considered good for embryo grading. The higher the number and letter grades are for an embryo, the better. It indicates how far along an embryo is in its developmental process, inferring which are best for transfer. Although both embryo grades are considered “excellent” with a 65% pregnancy rate and 50% live birth rate, 5aa is considered better because the blastocyst is partially or completely free from the zona pellucida and further along than an expanded blastocyst. Embryos receiving this grade are considered “average” and have a pregnancy rate of 50.3% and a 42/3% live birth rate. The blastocyst is expanded at this point and center around a blastocoel of any size with the ICM loosely grouped and the TE forming a loose epithelium.