Last updated: 8 October 2026
After eggs are fertilised in an IVF or ICSI cycle, embryologists observe each embryo at set times and record how it is developing. Many patients then receive a short code, such as "8-cell, grade 1" or "4AA", without a clear explanation. This guide works as a plain-language embryo grading chart: it explains the day 3 and blastocyst grading systems most laboratories use, how to read a blastocyst grade, and, just as importantly, what a grade cannot tell you about an individual embryo.
What embryo grading is, and why laboratories do it
Embryo grading is a structured description of an embryo's appearance under the microscope at a particular point in time. It records features such as the number and evenness of cells, the amount of fragmentation, and, at the blastocyst stage, how expanded the embryo is and how its two cell groups look. Grading helps the embryology team and your doctor decide which embryo to transfer first and which embryos are suitable for freezing.
Grading is not a measure of the embryo's genetic health, and it is not a prediction for any one embryo. It is a ranking tool that is most useful when comparing embryos from the same cycle.
When embryos are assessed: a day-by-day timeline
International consensus guidance from ESHRE and Alpha Scientists in Reproductive Medicine (the Istanbul consensus, first published in 2011 and updated in 2025) sets out standard observation times so that assessments are comparable. In simplified form:
| Day | What the embryologist looks for |
|---|---|
| Day 0 | Egg collection and insemination by conventional IVF or ICSI |
| Day 1 | Fertilisation check: a normally fertilised egg shows two pronuclei, one from the egg and one from the sperm |
| Day 2 | Early cleavage: an embryo developing as expected typically has around four cells |
| Day 3 | Cleavage stage: typically around eight cells; cell number, evenness and fragmentation are graded |
| Day 4 | Morula: the cells compact together into a tight ball |
| Day 5–6 | Blastocyst stage: a fluid-filled cavity forms, with an inner cell mass and an outer layer called the trophectoderm |
Not every embryo progresses through each stage. Some stop developing, which is a normal part of what culture in the laboratory reveals and is usually related to the embryo itself rather than to anything the patient did. Whether eggs are fertilised by conventional IVF or ICSI is decided before this stage; our guide on when ICSI is recommended explains how that choice is made.
Day 3 embryo grading chart (cleavage stage)
At the cleavage stage, embryologists look at three main features: the number of cells compared with what is expected for the day, whether the cells are of the size expected for that stage, and the proportion of the embryo taken up by fragments, which are small pieces of cell material that are not part of a cell. The presence of more than one nucleus in a cell (multinucleation) is also noted. The Istanbul consensus describes a three-tier scale:
| Grade | Fragmentation | Cell size | Multinucleation |
|---|---|---|---|
| Grade 1 (good) | Less than 10% | Stage-specific | Not seen |
| Grade 2 (fair) | 10–25% | Stage-specific for most cells | Not seen |
| Grade 3 (poor) | More than 25% | Not stage-specific | May be seen |
A day 3 report may therefore read, for example, "8 cells, grade 1". Laboratories differ slightly in the exact wording and scale they use, so ask your clinic how its own reports are written.
Blastocyst grading chart (day 5 and day 6)
A blastocyst is an embryo that has developed a fluid-filled cavity and two distinct groups of cells. The inner cell mass is the group expected to form the fetus. The trophectoderm is the outer layer expected to form the placenta and supporting tissues. The most widely used way of describing a blastocyst is the Gardner scale, which gives a number followed by two letters, such as 4AA, 4BB or 3BC.
The number: expansion stage
| Expansion | Meaning |
|---|---|
| 1 | Early blastocyst: the cavity is less than half the volume of the embryo |
| 2 | Blastocyst: the cavity is more than half the volume of the embryo |
| 3 | Full blastocyst: the cavity fills the embryo |
| 4 | Expanded blastocyst: the cavity is larger than the original embryo and the outer shell (zona pellucida) is thinning |
| 5 | Hatching blastocyst: the embryo has begun to emerge from the shell |
| 6 | Hatched blastocyst: the embryo has fully left the shell |
The first letter: inner cell mass
| Grade | Appearance |
|---|---|
| A | Many cells, tightly packed |
| B | Several cells, loosely grouped |
| C | Very few cells |
The second letter: trophectoderm
| Grade | Appearance |
|---|---|
| A | Many cells forming a cohesive layer |
| B | Few cells forming a loose layer |
| C | Very few, large cells |
Letters are usually assigned once a blastocyst has reached expansion stage 3 or beyond, because the two cell groups are difficult to distinguish earlier. Some laboratories use numbers (1 to 3) instead of letters, as described in the Istanbul consensus; the meaning is equivalent.
How to read common blastocyst grades
- 4AA: an expanded blastocyst with a tightly packed inner cell mass and a cohesive trophectoderm. This is a favourable appearance on the morphology scale.
- 4AB or 4BA: an expanded blastocyst in which one of the two cell groups is graded A and the other B.
- 3BB or 4BB: a full or expanded blastocyst with both cell groups graded B. Embryos with this appearance are commonly considered for transfer or freezing.
- Grades containing C: fewer cells in one or both groups. These embryos are generally ranked lower, but the decision about their use is individual and is discussed with your doctor.
- 5 or 6 with any letters: the embryo has begun hatching or has hatched; the letters still describe the two cell groups.
What embryo grades can and cannot tell you
- They can help rank embryos from the same cycle so that the one with the most favourable appearance is usually transferred first.
- They can guide which embryos are suitable for freezing.
- They cannot show whether an embryo has the normal number of chromosomes. An embryo with a high grade may not implant, and an embryo with a lower grade can still result in a healthy pregnancy.
- They cannot be compared precisely between clinics, because grading involves trained judgement and laboratories use slightly different scales.
- They do not stand alone. The outcome of a transfer also depends on the woman's age, the uterine lining and many other factors, so no grade guarantees a pregnancy.
Day 5 and day 6 blastocysts
Some embryos reach the blastocyst stage on day 5, and others on day 6. Embryos that form blastocysts on day 6 can still be suitable for transfer or freezing. The day of development is recorded alongside the grade, and your embryologist and doctor take both into account when deciding the order in which embryos are used.
How grades are used in decisions about transfer and freezing
In most cycles, the embryo with the most favourable combination of development and grade is chosen for transfer, and other suitable embryos are frozen by vitrification for possible later use. Your specialist will also discuss how many embryos to transfer. Transferring one embryo at a time reduces the risk of a multiple pregnancy, and India's ART (Regulation) Act, 2021 permits no more than three oocytes or embryos to be placed in the uterus in a treatment cycle. Our fertility preservation page explains consent and storage. If you are about to start treatment, our guides to preparing for an IVF cycle and the two-week wait after embryo transfer cover what happens before and after this step.
Embryo grading and genetic testing are different
Pre-implantation genetic testing (PGT) analyses a few cells from an embryo for specific genetic conditions. Under the ART Act, PGT may be used to screen embryos for known, pre-existing, heritable or genetic diseases only, and the Act prohibits any procedure intended to select the sex of a child. PGT is not a routine part of IVF and is discussed only where there is a clinical reason. The genetic testing section of our IVF & ICSI page explains how this is approached.
Time-lapse imaging and other add-ons
Some laboratories use time-lapse systems that photograph embryos at regular intervals, and the 2025 Istanbul consensus update addresses how such observations relate to standard assessment. The UK fertility regulator (HFEA) currently gives time-lapse imaging and incubation a grey rating, meaning there is not enough moderate- or high-quality evidence to rate whether it improves treatment outcomes. If an add-on is offered, ask what it is expected to change in your case and how it is charged.
Questions to ask about your embryos
- How many eggs were mature, and how many fertilised normally?
- On which day were the embryos assessed, and what grading scale does the laboratory use?
- Which embryo is recommended for transfer, and why?
- How many embryos are suitable for freezing, and on which day were they frozen?
- How will I be informed about embryo development during the culture period?
Embryology at our Kompally centre
Sree Nandaka IVF Centre has an on-site embryology laboratory beside the procedure theatre, with monitored incubators and vitrification for freezing eggs and embryos; see our centre. Our embryologists, Dr. Pooja Saripalla and Dr. Thrishika Pothan, work alongside our fertility specialists, including Dr. Deepa Rekha Koduru, Fertility Specialist (MBBS, DGO, DRM, MRM; Reg. No. 54076). Embryology updates are shared with you through your treating fertility doctor, who can explain what each grade means for your plan. Meet the team on our team page. For how embryo culture fits into a full cycle, see IVF treatment in Kompally and ICSI treatment in Kompally.
Frequently asked questions
Not necessarily. 4AA describes a favourable appearance, and such embryos are usually ranked first within a cycle. However, grading does not assess chromosomes, and the outcome also depends on age, the uterine lining and other factors. No embryo grade can guarantee a pregnancy.
Yes. Embryos with lower grades, such as BB or BC patterns, can implant and develop normally. Grades are used to rank embryos, not to rule out every embryo that is not top-graded. Your doctor will discuss whether a particular embryo is suitable for transfer or freezing.
It is a day 3 embryo with about eight cells of the expected size, little fragmentation (under 10% on the Istanbul scale) and no multinucleation. This is a favourable appearance for the cleavage stage.
It is common for some embryos to stop developing in culture, often because of factors within the embryo itself. Extended culture to the blastocyst stage reveals this before transfer. Your doctor can review the overall pattern of your cycle with you.
No. Most laboratories use the Istanbul consensus or Gardner principles, but wording and scales vary, and grading involves trained judgement. Grades are most meaningful when comparing embryos within the same laboratory.
After warming, embryologists check that the embryo has survived and is re-expanding before transfer. The original grade is recorded, and the appearance after warming is also assessed and explained by your treating doctor.
Book a consultation at our Kompally centre
Sree Nandaka IVF Centre, Plot 770, Jayabheri Park, behind Big Bazaar, Kompally, Hyderabad, Telangana 500014.
- Call +91 76720 01080
- Message us on WhatsApp +91 76720 01080 or email sreenandakafertility@gmail.com
- Book a visit online, or see contact details and directions
- Hours: Mon–Sat 8:00 am – 4:00 pm; Sunday 8:00 am – 2:00 pm
Please note. This article is general information, not medical advice and not a price list. What is appropriate for you depends on your history, your investigations and a specialist assessment. No fertility treatment can guarantee a pregnancy.
Sources
- ESHRE: Oocyte and embryo morphology assessment (Istanbul consensus update, 2025)
- Coticchio G, et al. The Istanbul consensus update. Human Reproduction 2025;40(6):989–1035
- Alpha Scientists in Reproductive Medicine & ESHRE SIG Embryology. The Istanbul consensus workshop on embryo assessment. Human Reproduction 2011
- Gardner DK, et al. Blastocyst score affects implantation and pregnancy outcome. Fertility and Sterility 2000
- HFEA: Treatment add-ons with limited evidence
- Assisted Reproductive Technology (Regulation) Act, 2021 — Gazette of India (PDF), Sections 24–26
Written by the Sree Nandaka IVF Centre editorial team, Kompally. General information only — not a substitute for a consultation.
