Understanding the Gardner blastocyst grading scale

December 18, 2011Carole 213 Comments »

Recently one of my readers had a question that I think many patients who go through IVF may have. She wanted to understand what blastocyst grading means in terms of embryo quality and then, obviously the implications for her  chances of pregnancy. Her question was: ” I just had two expanded blastocysts transferred on day 5.  Both were graded CC.  With my last IVF we did a SET with expanded blastocyst, grade BB that resulted in a chemical pregnancy.  In your mind is it more important that they are expanding blastocyst or the grade? (would an early blastocyst grade AA be better)….should we consider implanting on Day 3 (when we had 8 embryos still around)?”

Probably the most widely used blastocyst grading system is the David Gardner system which separately judges the functional milestones the embryo reached (namely how expanded the embryo is and its progress in hatching out from the zona pellucida (1-6 with 6 being completely hatched) and the number of cells in the inner cell mass (A-C) and trophectoderm layer (A-C where an A means the greatest number and most tightly organized cells).

What is a blastocyst? A blastocyst describes an embryo stage reached usually after about five days of development post-fertilization. It has about 50-150 cells and has started to develop specific regions with different cellular destinies. The blastocyst is working hard; pumping fluids towards its center, creating a fluid-filled center and expanding like a water filled balloon. The inner cell mass is a clump of cells  protruding into  the middle of the fluid-filled cavity. This inner cell mass will continue to grow and ultimately will be the source for all the cells of the future baby. The trophectoderm cells line the inner surface of the zona pellucida (the glycoprotein shell around the embryo) and play a supporting role, supplying cells to form the fetal part of the future placenta. So the grading system takes into account how much progress the embryo makes in hatching from the “shell” and also how richly endowed the inner cell mass and trophectoderm are in terms of cell number and quality. More expansion is better than less and more cells are better than fewer cells.

The expansion grade scale ranges from 1 (least expanded) to 6 (completely hatched).

Grade 1: the fluid-filled cavity takes up less than half the space of the embryo.

Grade 2: the fluid-filled cavity takes up more than half the space of the embryo.

Grade 3: the blastocyst cavity has expanded into the entire volume of the embryo, pressing the trophectoderm cells up tightly against the inside of the zona.

Grade 4: Expanded blastocyst, where the blastocyst has increased beyond the original volume of the embryo and caused the zona pellucida “shell” to become super thin.

Grade 5: Embryo has breached the zona and is hatching out of its shell

Grade 6: Embryo is completely hatched.

So the embryo is given a number grade (1-6), followed by a letter grade for the inner cell mass and then the trophectoderm (A,B or C).

For the inner cell mass:

A: Many cells, tightly packed

B: several cells, loosely packed

C:  very few cells

The trophectoderm grading goes like this:

A: many cells, forming a cohesive layer

B: Few cells, forming a loose layer

C: Very few large cells.

So a blastocyst with grade 5AA is partially hatched with many cells in the inner cell mass (cells for the future baby) and many tightly packed cells in the trophectoderm.

One thing to remember is the grading also represents a continuum. A healthy blastocyst starts out unhatched with a small space and few cells, then progresses to a hatching blast with many cells. When we pick a blastocyst to transfer on day 5 of culture, we preferentially pick the most advanced embryos that seem most “eager” to implant. That doesn’t mean that a blastocyst with a lesser grade won’t implant. They do. Remember we are looking at  a snap shot of embryo development and not all embryos in a group started the developmental pathway at the same instant.

A picture is worth a thousand words.

Here are a group of expanded but not yet hatched blastocysts (grade 4AA).










Here is a partially hatched blastocyst (grade 5AA). You can see the clear zona pellucida shell barely attached to one end of the blastocyst.









For more pictures of  graded blastocysts in various stages of development, look at these  (copyrighted) embryo pics  at this link  http://www.advancedfertility.com/blastocystimages.htm . These pics should help clarify the grading system.

Regarding my reader’s question of whether the degree of expansion or whether the cell number is more important in a successful implantation and pregnancy…well, it’s not usually a question of one or the other. In a healthy embryo, they tend to go together. Typically, as the cell number increases, so does the level of expansion.  What is a deal killer is if we don’t see a clump of cells corresponding to an inner cell mass because that means that none of the cells in the embryo have been allocated to make the future baby. Other than that, there is a lot of flexibility in the embryonic program and I wouldn’t despair if my blastocyst had a lesser grade. I have seen morulas (the stage before blastocyst) transferred on day 5 result in pregnancies so our preconceived notions of where an embryo must be on day 5 are not always predictive. However,  if an embryo is delayed by more than a day- say 8 cells on day 5, it probably will not be result in a pregnancy because it has likely stopped developing.

Hope that helps demystify embryo grading. 🙂 Best Wishes.



© 2011, Carole. All rights reserved.

213 Responses to this entry

  • jodie Says:

    hi carole,

    I am 43, and just gone through our first FET cycle in 7 yrs … we had a successful IVF in 2009 and got to freeze 6 A grade, Day 5 embryos ..

    This FET we transferred 2 embryo’s that were graded B and C (australian, and that’s all our clinic would tell us grading wise) …

    I am now 8dp5dt = 13dpo and still testing negative 🙁 here are our embryo’s pictures that were transferred on the 12th August 2016 … What can you tell us about them, if anything that is?

    thanks in advance



  • Carole Says:

    HI Jodie,
    I don’t even want to try to guess anything about the quality of the embryos from a photo that I did not take. The embryologist can roll the embryo around and look at it from all angles. You really should go back to them with any questions. What is more important is the chromosomal integrity and number of chromosomes. You were 36 when the embryos were created/frozen and you know that at least one of the embryo from the group were good – since your IVF was successful- so there may be another good one in the bunch- and it may not be possible to tell from the appearance. Based on your embryologists experience with similar aged/appearing embryos- I would transfer any others that meet their criteria for transfer. Good Luck!! Carole

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  • Anne Says:

    hi Carole – thank you so much for your informative website. I had a successful fresh cycle a little over three years ago and am preparing for a FET but I’m not sure whether my embryos are worth transferring 😐 My new clinic just received my embryos and the nurse there didn’t seem very encouraging about it. I was 31 when they were created and I learned their scores today – 3bb, 2bb, 3ba, 3ca. I think that is “fair” but not great. Of course the embryologist is on vacation, haha, but I am a little worried based on her reaction. Do you think I should consider doing a new cycle? I am now almost 35 🙁

  • Carole Says:

    Hi Anne, Unless the embryologist thinks the embryos are not alive, I think they are worth transferring because 1) appearance is a poor indicator of quality especially for “middlingg” embryos like yours and 2) embryos created at younger maternal ages are more likely to be good and implant than older embryos.Good Luck!! Carole

  • Carole Says:

    Hi Anonymous, Sometimes the eggs appear darker and that may suggest over-maturity. It is hard to determine the level of maturity from appearance. Sometimes egg membranes can be very fragile as well, and disintegrate easily on injection. Ovarian stimulation is critically important to maximize the number of mature eggs. If there is a another clinic nearby with as good or hopefully better pregnancy rates, you might consider trying another stim cycle with someone else. Check rates at http://www.sart.org. Good Luck! Carole

  • Carole Says:

    Hi Anonymous, Yes,unfortunately, appearance is a poor indicator of the chromosome number or integrity on the inside. In the course of normal IVF cycles, we would encounter abnormally fertilized eggs (having three pronuclei instead of two) at fertilization check. With the patients permission, we let those abnormal triploid fertilized eggs continue in culture and froze them at later stages of culture (day 3 or 5) . Generally, they made beautiful blastocysts and made it to day 5 so we were always careful to culture them separately from the normally fertilized eggs so there was no chance of confusing them- because you couldn’t tell the difference after fert check. We used these frozen abnormal embryos as teaching embryos for our technicians so they could become skilled at biopsying embryos for PGD cycles without risking any viable patient embryos. We had no shortage of test embryos so this was a common event. Don’t despair –there are many paths to parenthood if you are open to other options. Good Luck!!Carole

  • Leigh Says:

    Hi Carol,

    I recently completed a FET of a 6-day 4AA blast. My doctor informed me that the embryo was explanding and even commented on how it has expanded even more since he first looked at it.
    However, I felt despair when he handed me the pic, as it just looked like a cluster of cells with big distinguishable ICM. i know that you mentioned that you cannot determine much from a picture of an embryo, and that we don’t know the genetic integrity. But would you still be able to see an icm on a blastocyst that still expanding? Does a thawing embryo look like a indistinguishable ball of cells?


  • Anne Says:

    Hi Carole, we transferred two 3bb blastocysts two weeks ago and I found out today my FET was unsuccessful. Is it a red flag in your eyes for a 31 year old woman to have only one viable embryo out of 33 retrieved eggs, 17 fertilized with 7 making it to blastocyst? I am now about to turn 35 and feeling based on my prior cycle, my odds of achieving success are less than 3%. I feel like this must mean I have an egg quality issue, and I don’t think IVF can fix that. Feeling very blue about it all. Thank you for your input.

  • Anne Says:

    Oops – I thought of one additional follow up Q. My new lab tells me my highest graded blastocyst which I shipped from my prior clinic did not collapse during the vitrification process within one minute and it sat in the freezing fluid for 3-4 minutes. They are saying this is unusual and that it is probably not viable. Have you seen this? What do you think? I would like to be hopeful about it. Thank you again so much Carole! Kindest Regards.

  • Carole Says:

    Hi Leigh,
    I would not read too much into the embryo’s appearance after thaw, unless it looks completely dark and unexpanded. It is quite likely that the blastocyst is not fully expanded yet, but continued to do so after transfer, so please don’t worry. Good Luck!! Carole

  • Carole Says:

    Hi Anne,
    First, let’s see if your clinic is above average. Go to http://www.sart.org and find your clinic and look at it’s pregnancy and live birth rates and see if they are better than average–compare their rates to the national rate –also available at sart.org. IF they are not better than average, it increases the odds that it’s them and not you. I would expect more to get to blastocyst stage, but there are so many factors that affect that, it’s impossible to troubleshoot from outside the lab. But at least confirm that they are the best clinic you can get to. If not, it would be worth it to go to a better clinic and see how that goes. Also, this is an FET cycle- specifically look and see those rates for this clinic at http://www.sart.org. FET cycles also require good freezing and thawing skills. If they are new to this or using older methods, that can also make FET more tricky/less likely to work. Good Luck!! Carole

  • Carole Says:

    Hi Anne,
    It may mean that the embryo is unviable because it didn’t react to the change in osmotic environment as expected, so you may want to transfer the other ones first. The scoring is really based on a embryo beauty pageant and is often wrong. I wouldn’t read too much into it if the others received a lower score- they could very well be tougher and better at becoming babies. Good Luck!!Carole

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