Embryo Grading Explained: Day 3, Day 5 and What the Score Means

The honest picture, with numbers

An embryo grade answers one question: of the embryos you have, which should go back first. It does not answer the question everyone actually wants answered, which is whether this one will work. Holding those two apart is the single most useful thing to understand before the embryology call.

UK practice is not freelance on this. NG257 recommendation 1.49.5 tells clinics to evaluate embryo quality, at both cleavage and blastocyst stages, according to the Association of Reproductive and Clinical Scientists and UK National External Quality Assessment Service Embryo Grading Scheme. That is a named national scheme, which is why a British grade means something consistent between British clinics — and why a grade quoted by a clinic abroad, on a different scale, cannot simply be read across.

What the embryologist is actually looking at

Day 2 to 3: the cleavage stage

At this point the embryo is a small number of dividing cells and the HFEA lists what is assessed: the number of cells present, how fast the cells are dividing, whether the cell division is even, and whether there are any fragments of cells present, which indicates that some cells have degenerated. An embryo with the expected number of evenly sized cells and little fragmentation scores better than one that is lagging, uneven or fragmented.

Day 5: the blastocyst stage

By day five a viable embryo has organised itself into a fluid-filled structure with two distinguishable cell populations — an inner cell mass that becomes the fetus and an outer layer that becomes the placenta — and grading at this stage assesses how far that organisation has progressed and how well formed each part is. The extra days are the point: the HFEA explains that because blastocyst embryos have developed for longer in the laboratory, it is easier for the embryologist to select the embryos most likely to implant, and blastocyst transfers therefore tend to have higher birth rates.

In the UK, the HFEA states that roughly three quarters of patients have a blastocyst transfer and a quarter have a cleavage stage transfer.

Why your clinic may not go to day 5

Growing embryos on is a filter, and filters remove things. The HFEA is explicit that not all embryos left to the blastocyst stage will survive, that in some cases a couple could end up with no embryos available to transfer at all, and that there is no way of knowing whether an embryo that arrested on day four would have gone on to a successful pregnancy had it been transferred on day three.

That is why the recommendation is not universal. The HFEA notes you are more likely to be offered a blastocyst transfer if you are younger with a good number of embryos, and less likely if you are producing fewer healthy eggs, are older, or have only one or two embryos — because in those cases the filter risks leaving you with nothing to transfer.

What a grade does not tell you

Three things are worth being clear-eyed about.

First, grading is comparative. It ranks the embryos in front of the embryologist that morning. A top grade in a cohort of six means something different from a top grade in a cohort of two.

Second, grading is morphological. It describes how an embryo looks, not what its chromosomes are doing. This is precisely the gap that pre-implantation genetic testing claims to fill, and why that claim is contested — NG257 recommendation 1.48.1 says not to offer pre-implantation genetic testing for aneuploidy as part of fertility treatment to improve live birth rates.

Third, a lower-graded embryo is not a write-off. NG257 anticipates this directly: its transfer strategy distinguishes between cycles where one or more top-quality embryos are available and cycles where none are, and it still describes transferring what you have. The HFEA notes that rarely there may be no good quality embryos, in which case the doctor and embryologist make a judgement about whether any can realistically continue to a healthy pregnancy.

The technology sold on top of grading

Time-lapse imaging keeps embryos in an incubator with a built-in camera so development can be watched continuously rather than checked once a day, and it is frequently sold as a way to grade better. The HFEA rates time-lapse imaging and incubation black for improving the chances of having a baby, meaning that on balance the findings from moderate to high quality evidence show it has no effect on the treatment outcome. NG257's evidence review on embryo selection guided by continuous time-lapse sequence sits behind the same conclusion.

It may still be how a particular laboratory prefers to work, and stable incubation has its own logic. What it should not be is a line on your bill sold as a better chance.

What to do next

1. Ask which grading scheme the clinic uses. In the UK, NG257 points to the ARCS and UK NEQAS scheme.
2. Ask for the grades of all your embryos, not just the one being transferred. The ranking is more informative than the top score.
3. Ask why day 3 or day 5 has been chosen for you, and what the risk is of having nothing to transfer if you go on.
4. Ask how many embryos are being frozen and at what grade. NG257 recommendation 1.49.11 says to offer cryopreservation of remaining good-quality embryos.
5. Do not pay extra for time-lapse imaging on the basis of better selection. The HFEA rates it black for improving the chance of a baby.
6. Write the grades down. If this cycle does not work, they are part of the evidence for what to change next time.

When to seek help

If a cycle produces no transferable embryos, that is a medical event and not simply bad luck, and it should trigger a review consultation rather than an immediate repeat. Ask specifically whether the problem was egg number, egg maturity, fertilisation or development after fertilisation, because those four point to different changes. The HFEA lists failure of embryos to develop in the laboratory among the standard reasons a cycle is cancelled before transfer, and its counselling provision exists for exactly this conversation — clinics should offer support before, during and after treatment.