Embryo Grading - Q&A
Written by: Jessica Manns, MBMS, BS
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Description text goes heAll embryos begin as one cell and are known as a fertilized egg or zygote. From there, the cells inside of the embryo begin to divide in a process known as mitosis. Around the 3rd or 4th day of development, the cells in the embryo should morph together to form a structure called a morula. From there, a small cavity (blastocoel) will begin to form inside of the morula. At this point, the embryo enters the blastocyst stage of development and its cells become differentiated (destined to become different things). Over the course of 5-7 days, the embryo's cells should continue dividing until it consists of over 100 cells!re
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Most clinics begin grading embryos once they reach the blastocyst stage of development. Though embryo grading guidelines may differ between clinics, an embryo is typically graded based on its:
Stage of development (also called level of expansion)
Appearance of its inner cell mass (the group of cells that ultimately develops into the fetus)
Appearance of its trophectoderm (the cells which line the shell of the embryo that ultimately develops into the placenta and non-fetal structures)
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Clinics often use a numeric system called the Gardner scale to record an embryo's stage of development. The scale is from 1 to 6, see below.
Gardner Scale 1 (early blastocyst) = once the blastocoel forms inside of the morula; takes up <50% of embryo space
Gardner Scale 2 (blastocyst) = embryo’s cells continue to divide, and blastocoel also grows in size and consumes >50% of the embryo's space
Gardner Scale 3 (full blastocyst) = once the blastocyst consumes 100% of the embryo's space
The embryo has not grown in diameter but the cells inside of it have continued to divide
It should also have a clear inner cell mass and trophectoderm at this point
Gardner Scale 4 (expanded blastocyst) = blastocoel continues expand and the embryo begins to grow in diameter and "push on" its shell (zone pellucida)
This causes the zona pellucida to begin to thin out under the pressure of the expanding embryo
Gardner Scale 5 (hatching blastocyst) = a small hole begins to form in the zona pellucida and the embryo begins to hatch from this hole
Gardner Scale 6 (fully hatched blastocyst) = embryo will then fully hatch from its zona pellucida
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Clinics often use an alphabetical system to grade an embryo's inner cell mass (ICM) and trophectoderm (TE). While many clinics use an A-C scale, others use an A-D scale.
Inner cell masses are typically graded as follows:
A = present, compact, many cells, above average
B = present, fairly compact, average number of cells
C = present, not compact, fewer cells than average
D = not present or degenerate
Trophectoderms are typically graded as follows:
A = many cells, uniform in appearance, no gaps, above average
B = many cells, mostly uniform in appearance, small gaps, average
C = few cells, many gaps, not uniform in appearance, below average
D = no cells or degenerate
While "D" quality embryos are often not recommended for transfer, "C" quality embryos can be transferred and result in healthy live births. However, the poorer an embryo's grade, the lower its chance of success.
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Keep in mind that embryo grading is subjective, meaning that it can vary from person to person. And, again, each clinic may have its own grading system.
If your clinic's grading system differs from the system described above, reach out to your clinic for an explanation of its grading system.
Picture Examples
Left Image: this is a hatching embryo with a compact inner cell mass and above average trophectoderm. This embryo would be graded as a 5AA.
Right Image: this embryo is expanded (its zona pellucida is thin but it isn't hatching yet). It has a fairly compact inner cell mass in the 6 o clock position. The trophectoderm is present but not completely uniform and it has some gaps at the top. This embryo would be graded as a 4BB.
