Embryo Grading Explained: How Embryos Are Graded and Selected
Featuring Josh Kapfhamer, MD (Center for Reproductive Medicine), Eve Feinberg, MD (Northwestern University), Michael Baker, PhD (Aspire Fertility), and Gina McCain (Sanford Women’s Health).
If you are exploring embryo donation, you may come across embryo grades and wonder what they mean.
Grading is one of the tools a laboratory uses to describe an embryo’s appearance and stage of development. In the videos on this page, fertility providers and an embryologist explain how grading works, what the numbers and letters describe, and how clinics use grading alongside other information when choosing which embryo to transfer first.
What is embryo grading?
Embryo grading is a way for an embryologist to describe an embryo's stage of development and quality based on how it looks under the microscope. For blastocyst-stage embryos, a grade usually has two parts: a number that describes how far the blastocyst has expanded, and letters that describe the two groups of cells inside it. A common example is a grade like 4AA.
It helps to know that grading is descriptive, not a guarantee. As the providers featured here explain, an embryo’s grade is one input among several, and clinics consider it together with other information when planning a transfer.
What the numbers and letters mean
How Embryos Are Created Through IVF, Explained by an Embryologist
According to the embryologist featured, a blastocyst grade is read in three parts:
The number (1 to 6) describes the stage of the blastocyst. Early blastocysts are graded 1 or 2, a blastocyst is a 3, an expanded blastocyst is a 4, a blastocyst beginning to hatch from its outer shell is a 5, and a fully hatched blastocyst is a 6.
The first letter grades the inner cell mass, the tight ball of cells that goes on to form the fetus.
The second letter grades the trophectoderm, the outer layer of cells that goes on to form the placenta.
For the letters, A generally describes the strongest appearance, B describes a fair appearance, and C describes cells that are more sparse or uneven. So a grade like 4AA describes an expanded blastocyst with a strong inner cell mass and a strong outer layer.
Why grades can vary between laboratories
One point the providers make clearly is that the letter grades are somewhat subjective and can vary between laboratories and even between individual embryologists. One lab may reserve an A for only the most textbook-perfect embryos and grade most as B, while another may grade a wider range as A. Laboratories use quality control and training to keep grading consistent within their own systems, but the same embryo could be described a little differently in different labs. This is part of why a grade is best understood in conversation with the laboratory that assigned it.
How clinics use grading to select an embryo
Which Embryos Are Best? Embryo Grading and Blastocyst Selection Explained
The providers featured describe selecting an embryo for transfer in a sequence. They tend to look first at the stage of development, favoring the embryo that reached the furthest stage in the expected timeframe, and then use the letter grades as a way to distinguish among a group of otherwise strong embryos. As one provider puts it, once embryos have been judged good enough to freeze, they generally have real potential, and the grading becomes a way to decide the order of transfer rather than a pass or fail.
Other factors that matter alongside the grade
How Fertility Clinics Evaluate Donor Embryos: Grading, Quality, and Success Factors
Grading is only part of the picture. In the video, a fertility provider walks through several other factors a clinic weighs when evaluating embryos, including the age of the person who produced the eggs at the time of retrieval, the freezing method and technology used, how the embryos have been stored, whether they have undergone genetic testing, and whether a cohort of embryos has resulted in a pregnancy before. These factors are discussed in depth in the video, and your own clinic can explain how they apply to a specific set of embryos.
What this means for donor embryos
When intended parents are matched with donor embryos, the embryos are reviewed for suitability and eligibility, and the receiving clinic approves them before a transfer is planned. Grading and the other factors described above are part of how a clinic assesses the embryos in storage that a donor has offered. If you are considering embryo donation, your clinic can walk you through the grades and the full picture for the specific embryos involved.
Where genetic testing fits
Some embryos have undergone preimplantation genetic testing (PGT-A), and some have not. Testing is a separate topic from grading, and it comes with its own considerations. You can learn more on our page explaining what PGT-A is and how it works.
Have questions about donor embryos or the embryo donation process? Contact Embryo Connections and we will be glad to help.
The videos and information on this page are educational and reflect the perspectives of the providers and embryologist featured. They are not medical advice. Your own clinic and laboratory can advise you on the specific embryos in your situation.
Frequently Asked Questions
Q: What is embryo grading?
A: Embryo grading is how an embryologist describes an embryo’s stage of development and appearance under the microscope. For blastocysts, the grade usually combines a number for the stage of expansion with two letters describing the inner and outer groups of cells.
Q: What do the numbers and letters in an embryo grade mean?
A: The number (1 to 6) describes the blastocyst stage, from early blastocyst up to fully hatched. The first letter grades the inner cell mass and the second grades the outer trophectoderm layer, with A generally the strongest, then B, then C. A grade like 4AA describes an expanded blastocyst with strong cells in both groups.
Q: Does a higher grade guarantee success?
A: No. As the providers featured explain, grading is descriptive and is one factor among several. Clinics consider it alongside things like the age of the person who produced the eggs, the freezing method, and whether the embryos were genetically tested. Your clinic can explain what the grades mean for a specific set of embryos.
Q: Why do embryo grades vary between laboratories?
A: The letter grades are somewhat subjective and can differ between labs and technicians. Labs use quality control to stay consistent internally, but the same embryo might be described a little differently elsewhere, which is why a grade is best understood in conversation with the lab that assigned it.
Q: How does grading apply to donor embryos?
A: When intended parents match with donor embryos, the embryos are reviewed for suitability and the receiving clinic approves them before a transfer is planned. Grading is part of how a clinic assesses the embryos a donor has in storage.
Have questions about donor embryos or the embryo donation process? Contact Embryo Connections and we will be glad to help.