Decoding embryo grading, and why it matters

June 29, 2023

Decoding Embryo Grading… and Why it Matters 


4AA, 3BA, 6BC…what does it all mean?

While embryo grading can feel like deciphering a secret code, it is actually a relatively simple system to learn. When you understand the code, you’ll have a better grasp of the quality of embryos you are either donating or receiving.  To note, every lab and embryologist grades a little differently, just like teachers, so embryologists may grade the same embryo differently. 

What is Grading?

The embryo grading system is a way to evaluate the quality of an embryo created through in vitro fertilization. This system is based on the embryo’s appearance under a microscope. Scientists look for certain characteristics that indicate whether the embryo is healthy and likely to develop into a fetus under the right conditions, or if there are indications that could make continued development less likely. 

How Embryos are Graded

While different clinics have slightly different grading systems, most are looking at the number of cells, the symmetry of the cells, the presence of fragmentation, and the quality of the zona pellucida (the outer shell of the embryo). This information helps develop a “grade” for the embryo that can suggest whether or not the embryo is a good candidate for implantation. 

Embryos are typically observed and graded on days 3 and 5. This captures two key developmental stages for the embryos:

  • Day 3 is the “cleavage stage” where we are able to clearly see the embryo’s cells and the quality of their structure. On Day 3, an embryo may be given a numerical score based on how many cells are visible, generally from 6 – 8, with 8 being the best. 
  • On Day 5, we can see the “blastocyst stage.” At this point, we can see how well the embryo is developing in size as it prepares to “hatch.”  Embryos are sometimes not fully expanded on Day 5, so an embryologist may choose to extend culture to Day 6 and even to Day 7 to assess if those smaller embryos continue to expand into healthy blastocysts.

Decoding the Embryo Blastocyst Grading System 

Many embryos are graded using a Gardner or Modified Gardner grading scale, which generally consists of a number and a letter (such as 5AB). Here’s what they mean: 

  • The Number
    On Day 5 (and Day 6 or Day 7), embryos are given a number between 1 and 6. This refers to how far along the blastocyst is in its development: 
Number Alternate Grading Definition
1 Early blastocyst: Fills less than half the volume of the embryo
2 Blastocyst: Fills about half the volume of the embryo
3 EB Expanding Blastocyst
4 XB Expanded Blastocyst: Blastocyst is expanding beyond the size of the early embryo and the zona pellucida, its shell, is thinning
5 HG Hatching Blastocyst: The blastocyst has started to hatch from the zona pellucida
6 H Hatched blastocyst: The blastocyst has completely escaped the zona pellucida

The ideal expansion for a day 5 embryo is expanded, hatching, or hatched. This indicates that the blastocyst has properly developed and is ready to implant. Day 5 Expanding blasts also have good success rates.

  • The Letters
    Following the expansion number are two letters. In a Gardner grading scale, they are A, B, or C. The Modified Gardner scale includes A, B, C, and D, which allows for a bit more distinction of the embryos across the same spectrum. The first letter represents the quality of the ICM (or inner cell mass). This is the cell mass that continues to develop into the fetus. The second letter is the TE (trophectoderm), which becomes the placenta and other tissues that support the fetus.

    ‘A’ is the highest grade, meaning that the cells are dense and uniform within the cavity, in a cobblestone formation. ‘B’ means there are several cells in the cavity starting to become dense, though varying in size. and ‘C’ means there are few cells in the structure, enough to consider the embryo viable. Grades below C on a Gardner scale and D on a Modified Gardner scale are not considered viable.  
  • Fragmentation score
    Some clinics will also share a fragmentation score, usually as a percentage. Fragmentation indicates an uneven division of the cells – the higher the percentage, the lower the chance of a healthy pregnancy. Ideally, an embryo will have a fragmentation score of less than 10%. Generally, a clinic will only share this information if fragmentation is outside of a comfortable range. 
  • Pre-implantation Genetic Testing (PGT)
    In addition to grading, some patients choose to biopsy and test their embryos to ensure  that embryos are chromosomally normal (PGT-A, PGS, CCS), and/or to rule out specific genetic conditions carried by one or both genetic contributors (PGD, PGT-M). Click here to learn more about PGT testing and why it matters. 

The Bottom Line 

Embryo grading can be an extremely helpful way to understand an embryo’s development and overall quality. But remember – this grading system is just a tool we use to help us make sense of nature. Nature often takes her own course! A 5AA grade does not guarantee pregnancy, and a lower-graded embryo could easily become a healthy, thriving baby. 

Use embryo grading to make informed decisions along your embryo donation journey, but don’t lose hope if an embryo doesn’t have a “perfect” grade.