Embryo Grading Explained: What Day 3 and Day 5 Grades Mean for Your Transfer

An embryo grade is the embryologist’s written description of how an embryo looks under the microscope on a particular day: how many cells it has, how evenly it has divided, and, by day 5, how well its two cell groups have formed. It is a way of ranking embryos so the most promising one is transferred first. It is not a score for the baby, and it says nothing about chromosomes. I did not understand that during our first round, when a single letter on a phone call from the lab could end my day, so this is the explanation I would have liked on the morning of the call.

What an embryo grade actually is

A grade is a snapshot of appearance, taken at fixed points in the days after fertilisation. In a standard cycle the eggs are fertilised in the lab and then left to develop for up to six days before the best is transferred and any other good-quality ones are frozen¹. The embryologist checks them at set times and records what they see, and those records are the grades you are given. Where this sits in the whole sequence is set out in the site’s guide to egg collection and embryo transfer.

Two things follow from that definition. First, a grade can change from one day to the next, because embryos develop at different speeds and a slow starter can catch up (or a strong starter can stop). Second, a grade is a judgement made by a person looking down a microscope, using a scale their lab has chosen. Different labs use different scales, and two embryologists in the same lab can reasonably disagree by a grade, which is worth remembering before you compare your numbers with a stranger’s.

Day 3 grading: cells, symmetry, fragmentation

On day 3 an embryo is graded on three things: how many cells it has, whether the cells are of similar size, and how much fragmentation there is. By day 3 an embryo that is developing on time usually has somewhere around 6 to 10 cells, with 8 being the textbook figure. Fewer than 6 is generally read as slow, though slow is not the same as finished.

Symmetry means whether the cells look roughly equal. Fragmentation refers to small pieces of cell material that have broken off during division; a little is common and does not seem to matter much, while heavy fragmentation, often described as more than 25 percent of the embryo’s volume, is associated with a lower chance of implantation. Clinics express the day 3 result in different ways, for instance “8 cell grade 1” or “8A”, and the grade letters or numbers run in whichever direction the lab has chosen, so always check which end of the scale is good.

Day 5 grading: the blastocyst and its three scores

By day 5 or 6, an embryo that is still developing has usually become a blastocyst: a fluid-filled ball with two distinct groups of cells. The inner cell mass is the cluster that will become the baby. The trophectoderm is the ring of cells around the outside that will become the placenta. Most clinics grade blastocysts with a version of the Gardner system, which gives one number and two letters.

• The number, 1 to 6, describes expansion: how large the fluid cavity has become and whether the embryo has started to hatch out of its outer shell. A 1 or 2 is an early blastocyst, a 3 is a full blastocyst, a 4 is expanded, a 5 has begun to hatch and a 6 has hatched completely.
• The first letter, A to C, grades the inner cell mass: A for many tightly packed cells, B for several loosely grouped cells, C for very few.
• The second letter, A to C, grades the trophectoderm: A for many cells forming a neat cohesive layer, B for fewer, looser cells, C for very few large cells.

So a 4AA is an expanded blastocyst with good scores for both cell groups, a 3BB is a full blastocyst with average scores, and a 5BC has started to hatch with a reasonable inner cell mass and a thin outer layer. In practice most embryologists regard anything from about 3BB upwards as good quality, and many labs will freeze embryos down to a BC or CB grade, depending on their policy. Embryos that have not formed a blastocyst by day 6 are generally not kept.

Some clinics use a simpler scheme instead, such as a single score from 1 to 5 or the words “top”, “good”, “fair” and “poor”. None of these systems is compulsory or official; they are lab conventions, which is why the same embryo can carry different labels in different places.

How much grade predicts success

Higher grades are linked with a somewhat better chance of pregnancy per transfer, and that is the whole reason grading exists: it lets the lab put embryos in a sensible order. But the link is a tendency across thousands of transfers, not a rule for your embryo. Blastocysts graded B and C implant and become healthy babies regularly, and top-grade embryos fail regularly too. What matters far more than the letters is the age of the eggs, for the reasons set out in age and fertility: a 3BB from eggs collected at 32 carries a better outlook than a 4AA from eggs collected at 42, because the strongest predictor of an embryo’s chromosomes is the age of the woman whose eggs they were.

That is the second thing a grade cannot tell you. Appearance and chromosome count are only loosely related. A beautiful blastocyst can have the wrong number of chromosomes, and a plain-looking one can be entirely normal. The only way to check chromosomes is to biopsy a few trophectoderm cells and test them (PGT-A), which is an optional extra with its own costs and limitations, and which the regulator rates separately from grading². Our guide to IVF add-ons covers when that test is and is not worth considering. Time-lapse imaging, another common extra, is essentially continuous grading: the same visual information, watched without opening the incubator.

The figure I would hold on to is this: per transfer, a good-quality single blastocyst gives many people a chance in the region of a third to a half at younger ages, falling with age, and the grade shifts that figure at the margins rather than deciding it. The way those per-transfer numbers are counted, and why cumulative chances across several transfers are the more useful frame, is explained in IVF success rates.

Why the numbers fall so steeply over the week

The single most upsetting part of the lab week for us was watching the count drop. Twelve eggs became eight mature ones, became six fertilised, became three on day 3, became two blastocysts. That was a completely ordinary week. The attrition is built into the biology, not into the lab.

In round terms, about 60 to 70 percent of mature eggs fertilise normally with either standard IVF or ICSI, and roughly 30 to 50 percent of those normally fertilised eggs go on to form a blastocyst by day 5 or 6. So from ten mature eggs, six or seven fertilised and two or three blastocysts is the ordinary shape of a cycle, and the range around it is wide: some people get five or six from the same starting number, some get one, and some get none. Age, the number of eggs collected, sperm quality and lab conditions all move the figure³. The reason the lab lets embryos grow to day 5 at all is that the ones which stop before then would very likely have stopped inside you as well; the blastocyst stage is a filter that removes embryos which were never going to implant, so the two that remain are a much stronger pair than the six they came from.

The trade-off is the risk of reaching day 5 with nothing. When only one or two embryos are developing, some clinics prefer to transfer on day 3 rather than risk that outcome, and there is a reasonable argument either way. It is a decision your embryologist will talk you through, and it depends on your numbers, not on a general rule.

What our grades meant in the end

On our second round, the one that failed, we transferred fresh the best blastocyst we had ever made, a 4AA that the embryologist described, with real warmth, as textbook, and froze two slower ones. The embryo that became our daughter was one of those two: a day 6 blastocyst that had lagged the whole week and was frozen as a 4BB, then thawed for the frozen transfer that made up our third round. I am not telling you that story because it proves anything; it does not, and I could easily find someone whose experience ran the opposite way. I am telling it because a grade is a number about a group of embryos and not a prophecy about yours, and I wish I had believed that on the morning I cried in the car about a letter B.

It also changed how I heard the daily calls. The lab was not grading me. It was sorting, and the sorting was in my favour, because it meant the embryo they chose to transfer was the one with the best chance on the evidence they had.

Freezing, thawing and grades

Grades matter for freezing in two ways. First, most labs only freeze embryos above a certain grade, because very poor-quality embryos are unlikely to survive thawing or to implant; that is why you may end a cycle with fewer frozen embryos than you had blastocysts. Second, thawing is a second check: the embryologist looks at the embryo again after it is warmed and tells you whether it has survived intact and, often, whether it has re-expanded. Modern vitrification means the large majority of good-quality embryos survive thawing⁴, and a frozen embryo transfer of a good blastocyst now performs broadly as well as a fresh one.

Clinics generally transfer one embryo at a time, and grades are part of how they choose which one. Transferring the single best embryo rather than two is what has driven the fall in multiple births from IVF, from roughly one in four in the late 2000s to under one in ten in the UK today¹, and a single good blastocyst is the usual recommendation for most people under 40 with good-quality embryos⁵.

Questions worth asking your embryologist

If the grades are troubling you, these are the questions that turned the numbers into something I could use:

• Which grading system does this lab use, and which end of the scale is good?
• Where does this embryo sit within that scale for someone my age?
• Why did you choose this one to transfer first?
• Will the others be frozen, and what is your threshold for freezing?
• If we reach day 5 with only one or two embryos, would you consider a day 3 transfer?

Embryologists are used to these questions and most are glad to be asked; the daily call is short because there are many to make, not because there is nothing to say.

This is general information and not advice about your embryos. Grading scales and freezing policies differ from lab to lab, and your own embryologist and fertility team are the right people to explain what your particular grades mean and what they suggest for your next step.