Embryo grading is a tool that helps embryologists and physicians during an IVF treatment to determine, exactly which embryos to transfer, the optimal day of transfer and the appropriate number of embryos to transfer. Typically done on Day 3 and Day 5, grading is not used independently, but is just one part of a complex process that is used along with many factors like patient’s age, fertility history, and other information for deciding the optimum embryo(s) to transfer to maximize the chances of successful pregnancy and minimizing the risks multiple pregnancies.
While embryo grading provides valuable and critical information, it is important to note that even lower-graded embryos can sometimes result in successful pregnancies. Grading does not guarantee pregnancy outcomes, as other factors such as uterine receptivity and genetic abnormalities can also influence success
Grading typically only refers to the “appearance of the embryo” and as individual judgement is important; this is a highly subjective assessment. At GENESIS, all assessments are done on inverted microscopes; using CAP proficiency testing 3 times a year to verify accurate and standardized embryo quality assessment.
While embryo grading provides valuable and critical information, it is important to note that even lower-graded embryos can sometimes result in successful pregnancies. Additionally, grading does not guarantee pregnancy outcomes, as other factors such as uterine receptivity and genetic abnormalities can also influence success
Day 3 Embryos
Day 3 (D3) embryos are referred to as “cleavage stage embryos”. This means they are dividing (or cleaving) but the embryo itself is not growing in size. They typically consist of 6-8 cells inside an outer covering called the zona pellucida; also known as the shell. GENESIS uses cell number and degree of fragmentation to assess the quality of D3 embryos. Typically, a higher number of cells with lower fragmentation is considered more favorable, for e.g. 8A on D3 is the best grade.
These embryos show that there are 6-8 evenly sized cells, with no or less than 10% fragmentation.
These embryos have more uneven or irregularly shaped cells with 25-50% fragmentation.
These embryos show 50% or more fragmentation.
Day 5 Embryos
Day 5 (D5) or blastocyst grading is more standard, as most clinics use the Gardner and Schoolcraft’s three part scoring system. By day 5 embryos should have started to outgrow the tight confines of the zona pellucida, or shell.
Blastocysts are graded on three factors:
- Degree of expansion based on how expanded the cavity is. This is graded on a scale of 1-6 with 6 being the most expanded
- Appearance of the inner cell mass (the part that makes a baby) which is graded for the compaction of cells with either an A, B, or C with A being the best.
- Appearance of the trophectoderm (the part that makes the placenta) also graded for the number of cells with A, B or C; with A being the best.
Each of those factors is combined to create the grade (number, letter, letter).
Blastocyst Grade per factor
The first part of the grade is a number. The number represents the expansion of the embryo cavity.
- Blastocoel cavity is 1/3 the volume of the embryo
- Blastocoel cavity is 1/2 the volume of the embryo
- Full blastocyst is 80%, completely filling the embryo
- Expanded blastocyst, the cavity is larger than the embryo; the zona is thinning
- Expanded blastocyst, beginning to hatch out of the zona
- Expanded blastocyst, fully emerged from the zona
The second part of the grade is a letter. The first letter following the number represents the inner cell mass, or ICM quality.
A. Many cells, tightly packed
B. Several cells, loosely grouped
C. Very few cells
The third part of the grade is another letter. The second letter represents the quality of the trophectoderm. This is the cellular layer that makes the placenta and surrounding membranes.
A. Many cells, forming a cohesive epithelium
B. Few cells, forming a loose epithelium
C. Very few large cells
Typically, an expanded blastocyst with a well-developed ICM, which forms the fetus, and TE, which becomes the placenta, are indicators of a healthy embryo. For example, a 5AA on D5 is the best grade.
How important is embryo grading?
To determine whether an embryo has good potential or not, all of the components of the embryo must be taken into account. While an A grade is better than a D grade, embryos continue to develop. Because of this, their grading can change; eventually making lower graded embryos candidates for embryo transfer or freezing.
Embryo grading is a tool to help scientists and physicians; but it is only one part of the decision process. This is why our physicians and scientists determine the potential of an embryo by taking into account not only the grading, but all of a patient’s unique medical history.
Ref: Davis Gardner, Colorado Center for Reproductive Medicine
Original post April 7, 2021
Updated August 16, 2023