Embryo grading explained: blastocyst numbers, letters, and what morphology can actually predict
A grade like 4AB is a structured description of how an embryo looked on a given day. It helps rank embryos for transfer, but it cannot see chromosomes or guarantee implantation.
Your clinic messages to say you have "two 4AA blastocysts and one 5BC." It looks like a test result, and it is natural to read it like a school report. It is better understood as a short visual description. An embryologist looked at each embryo under a microscope at a set time and recorded how far it had developed and how its two main cell groups looked. Clinics use that description to decide which embryo to transfer first and which to freeze.
This guide decodes the common blastocyst grade and the simpler day-3 assessment. It explains what the 2025 international consensus on embryo assessment changed, and is honest about what a grade can and cannot tell you.
Decoding a blastocyst grade
A blastocyst is an embryo that has formed a fluid-filled cavity, usually on day 5, 6 or occasionally 7 after fertilization. A grade such as 4AB has three parts:
| Part | What it describes | How to read it |
|---|---|---|
| Number (e.g. 4) | Expansion: how far the cavity has grown and whether the embryo has started to hatch from its shell | Higher numbers mean a more advanced stage at the moment of observation. It is a stage, not a quality score. |
| First letter (e.g. A) | Inner cell mass (ICM): the cluster of cells that will form the fetus | A tight, well-defined cluster scores higher than a loose or sparse one |
| Second letter (e.g. B) | Trophectoderm (TE): the outer layer that will form much of the placenta | Many cells forming a cohesive layer score higher than few, uneven cells |
The letters are graded separately because the two cell groups do different jobs. An embryo can have an excellent inner cell mass and a weaker outer layer, or the reverse. Labs differ slightly in how they weigh one against the other when ranking.
The new "D" grade
The 2025 consensus adds a D category for blastocysts showing degenerative features or with no distinct inner cell mass (ESHRE, 2025). If your lab has adopted the update, a D is the grade most likely to mean an embryo will not be used. A C is not automatically discarded.
Day 5, 6 or 7: timing is part of the grade
When a blastocyst forms is information in its own right. Many labs give some priority to blastocysts that form earlier when two embryos look similar, and each lab sets its own ranking rule. Slower embryos are not written off, though. The updated consensus describes day-7 blastocysts as potentially viable and possibly suitable for clinical use, which matters for anyone whose embryos tend to develop slowly. If your report does not say which day each blastocyst was graded or frozen, ask. "4AA on day 6" and "4AA on day 5" are not the same entry. Our day-by-day guide to embryo development explains what normally happens at each stage.
Day-3 embryos are graded differently
If embryos are assessed or transferred at the cleavage stage (day 2 or 3), there is no cavity, ICM or TE to grade. Instead embryologists look at the number of cells, how evenly sized they are, how much fragmentation there is, and whether each cell has a single nucleus. The 2025 consensus says embryos with under 10% fragmentation, one nucleus per cell and cell sizes appropriate to their stage are prioritised. It gives reference timings too: under ICSI, the median embryo reaches four cells about 38 hours after insemination and eight cells about 57 hours after, with conventionally inseminated embryos running about an hour behind (ESHRE, 2025).
A report might say "8-cell, grade 1" or "7-cell, 10–25% fragmentation." Different labs use different shorthand, so ask for the key if it is not included. Whether to transfer at day 3 or day 5 is a separate decision, covered in our day 3 vs day 5 transfer guide.
Before that: the fertilization check
The first assessment happens the morning after the eggs are inseminated. The consensus recommends checking fertilization 16 to 17 hours after insemination. Normal fertilization shows up as two small nuclear outlines, written 2PN, which carry the maternal and paternal chromosome sets; the consensus keeps that as the preferred pattern. Zygotes with one pronucleus, or other unusual patterns, may be cautiously considered if PGT-A is used to confirm a normal chromosome set. That is why some reports carry a note against an embryo's ID from day 1.
What a grade cannot tell you
- Chromosome status. Grading looks at shape and development. It cannot see whether an embryo has the right number of chromosomes. A top-graded blastocyst can be aneuploid, and a modest one can be euploid. Genetic testing is a separate step with its own report.
- A guarantee. Higher grades have better average outcomes across many embryos. For one embryo, the grade shifts the odds; it does not decide them. Lower-graded embryos do implant and lead to healthy births, which is why the consensus now describes C-graded blastocysts as potentially viable.
- The health of a future child. A grade describes the embryo at one moment in the lab, nothing more.
Why two labs might grade the same embryo differently
Grading is done by trained people looking at living, changing cells. The embryo may look slightly different an hour later, and two experienced embryologists can make different calls on a borderline B versus C. The Istanbul Consensus exists largely to reduce that variation by agreeing shared definitions and observation times. Not every lab follows it exactly, though, and some use their own scales. Grades from different clinics are not always directly comparable, which matters if you have embryos stored in more than one place.
Does time-lapse imaging make grading better?
Time-lapse incubators photograph embryos continuously, so embryologists can see the timing of each division without taking embryos out. That gives a richer record. As a way of improving the chance of a baby, though, the UK regulator rates time-lapse imaging and incubation "black": moderate to high-quality evidence shows no effect on that outcome (HFEA add-ons ratings, reviewed 2026). The consensus mentions AI-based assessment as a possible future route to more objective grading, but does not present it as established.
How clinics actually use grades
In practice, grades drive three decisions:
- Transfer order. Usually the highest-ranked embryo goes first, using grade and day together. The 2025 consensus provides ranking guidance for fresh and frozen transfer.
- Freezing. Each lab sets a minimum standard for which blastocysts are worth vitrifying. Ask what yours is.
- Biopsy. If you are having genetic testing, an embryo needs enough trophectoderm cells to sample, so very early or poorly expanded blastocysts may be biopsied later or not at all.
If embryos have also been genetically tested, morphology is used to rank the euploid embryos among themselves. If you are learning to read those results, our guide to reading a PGT embryo report covers the genetic side.
An illustrative embryo summary
The table below is illustrative only, to show how grade and timing combine. It is not real patient data.
| Embryo | Day frozen | Grade | How a lab might read it |
|---|---|---|---|
| E1 | Day 5 | 4AA | Fully formed, strong ICM and TE. Likely first in line. |
| E2 | Day 5 | 4BB | Good on both counts. A reasonable next choice. |
| E3 | Day 6 | 5AB | Hatching, strong ICM, slower to form. Ranked by lab policy. |
| E4 | Day 6 | 3BC | Weaker outer layer. Still potentially viable under the 2025 consensus. |
Questions to ask about your grades
- Which grading system do you use, and have you adopted the 2025 Istanbul update?
- On which day was each embryo graded and frozen?
- How do you rank day-5 against day-6 embryos of similar grade?
- What is the minimum grade you freeze, and what happens to embryos below it?
- If my embryos are tested, how will grade and genetic result be combined to choose transfer order?
A grade tells you how an embryo looked on a given day. It helps rank embryos. It cannot promise an outcome.
FAQ
Is a 5AA better than a 4AA?
Not necessarily. The number describes how expanded the blastocyst was when it was observed, not its quality. A 5 may simply have been seen slightly later in its development. The letters, the day and your lab's own data matter more for ranking.
Should I discard embryos graded C?
Not on the grade alone. The 2025 ESHRE/ALPHA consensus describes C-graded blastocysts as potentially viable and possibly suitable for clinical use. Discuss your lab's freezing and transfer policy before making disposition decisions.
Why did my embryo's grade change between day 5 and day 6?
Embryos keep developing, so a blastocyst graded early may expand further or change appearance a day later. Labs usually record the grade at the time of freezing or transfer.
Does a frozen embryo keep its grade after warming?
The grade on your report reflects its appearance before freezing. After warming, embryologists also check that it survived and re-expanded, which they may note separately on the day of transfer.
Can I get photos of my embryos?
Many clinics share images or time-lapse clips if you ask. They can be meaningful to have, but a single image cannot show chromosome status or predict outcome, so use them as a keepsake rather than a diagnostic tool.
Planning treatment abroad?
Planning to grow or store embryos abroad? Ask any clinic you are considering which grading system its lab uses before you compare results. For destination and logistics planning, see IVFAbroad.co; for Colombia, ColombianIVF.com and ColombiaMedical.co.
Ask about Colombia