In Vitro Fertilization (IVF)

Embryo Grading

Embryo Grading

Many patients undergoing in vitro fertilization (IVF) commonly ask questions about embryo grading, from, “What is the quality of my embryo(s)?” or “What is the grade of my embryo(s)?” and “What are my chances of a successful pregnancy?”

What Does Grading of Embryos Mean?

Grading of embryos is a subjective, observational assessment made by the embryologist based upon experience and training. The grade or score is believed to reflect the general quality and potential of the embryo. It is like a teacher grading a paper in that it is an opinion of quality. It is based mainly on the morphological appearance of the embryo at a particular point in time. Embryological studies of the past have provided benchmark or “textbook” examples of embryos of different qualities and stages, which are generally thought of as comparative models to be used against when grading other embryos.

For blastocyst-stage embryos, a system has been created and adopted by IVF laboratories that quickly provides information regarding the quality of the embryo. As the embryo grows, it exponentially divides its cells, eventually becoming the blastocyst stage embryo 4 to 5 days after fertilization. At this point, the embryo can be described as appearing akin to a “hollow soccer ball”. It should have many cells and have expanded in size. The cells at this stage should define themselves as a grouping of cells within the blastocyst called the inner cell mass or ICM, as well as cells that compose the outer perimeter of the expanding embryo, the trophectoderm.

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What is Considered Good Embryo Quality?

When the embryologist scores the embryo at this stage, the degree of expansion and cellular composition will be assessed and graded. To convey this information, a system of numbers and letters is used. Expansion is described first, with 1-3 (early blastocyst) being the least expanded and 4-5 (expanded blastocyst) the most expanded. The cell composition of the inner cell mass is described next, with an “A” representing high quality and a “C” or “D” representing poorer quality. A tight, compact grouping of well-defined cells composing the ICM would be graded as an A-good quality. A poorly defined grouping of fewer cells would result in a poorer grade. Lastly, the outer cells of the trophectoderm are scored. Once again, many well-defined cells would be highly graded while fewer cells of lesser quality would be downgraded. Thus, an ideal day 5 embryo for embryo transfer or freezing would be scored as 4AA.

Variations of the system may be proprietary to a laboratory but convey the same information. At Fertility Centers of New England, our laboratory scoring uses a descriptive expansion EB, B, or EXB (early blastocyst, blastocyst, expanded blastocyst) and cell composition listed as a numerical score 1-4, with a score of 1 being best. Our scoring of an EXBl-1 blastocyst embryo would be equal in grade to a 4AA grading system (often referred to as the “Gardner grading system.”) A correlation and hierarchy of grading from best to worst would be:

FHB1-1 6AA Fully hatched blastocyst Excellent
HB1-1 5AA Partially hatched blastocyst Excellent
EXB1-1 4AA Fully expanded blastocyst Excellent
FHB1-2; FHB2-1 6AB; 6BA Slight cell variation Very Good
HB1-2; HB2-1 5AB; 5BA Slight cell variation Very Good
EXB1-2; EXB2-1 4AB; 4BA Slight cell variation Very Good
B1-1; B1-2; B2-1 3AA; 3AB; 3BA Slight cell variation; mid-expansion Good
FHB2-2; HB2-2; EXB2-2 6BB; 5BB; 4BB Lower cell quality; fewer cells Fair
FHB2-3; FHB3-2;
HB2-3; HB3-2
EXB2-3; EXB3-2
6BC; 6CB; 5BC;
5CB; 4BC; 4CB
Noticeable deficiency in cell number or composition of ICM and/or trophectoderm Fair / Poor
EB 2BB-2CC Early blastocyst; delayed Fair / Poor


Does Embryo Quality Affect IVF Success?

Top and second tier embryo grading is considered very good embryo quality or grade. These embryos all qualify as embryo transfer candidates or for cryopreservation. As we progress further into the lower tiers, confidence in the potential of those embryos decreases as these embryos, though progressing, exhibit characteristics that may not be favorable for a successful outcome. Lowest-grade embryos typically do not result in a successful pregnancy and have higher rates of chromosome abnormalities (aneuploidy) and a higher chance of arrested development. For these reasons, lower-grade (fourth-tier) embryos are neither selected for transfer nor cryopreserved (frozen).

Contact Us

Your Fertility Centers of New England fertility doctor reviews all results with you.  If you are not currently a patient and have more questions on embryo grading or other concerns about your fertility, please contact us for an initial consultation. We are here to help.

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