FAQ: Understanding Embryo Grading in IVF
The Key Takeaway: Embryo grades are descriptive, not predictive. They help embryologists communicate what they see under the microscope, but they cannot tell you whether an embryo will become a healthy baby. In 2025, the guidelines for embryo assessment were updated (The Working Group on the Update of the ESHRE/ALPHA Istanbul Consensus et al., 2025). This is what is most commonly used by clinics we work with.
1. What does a blastocyst grade like “4AB” actually mean?
A blastocyst grade has one number + two letters, each describing a different part of the embryo.
Blastocyst expansion — the number (1–6) shows how expanded the embryo is
1–2: early blastocyst
3: full blastocyst
4: expanded blastocyst
5: hatching blastocyst
6: fully hatched blastocyst
Inner cell mass (ICM) — the first letter (A, B, C) describes the cluster of cells that will become the foetus
A: tightly packed, many cells
B: looser group, moderate cells
C: fewer cells, more spread out
Trophectoderm (TE) — the second letter (A, B, C) describes the outer layer that becomes the placenta
A: many, even cells
B: slightly uneven or fewer cells
C: fewer, uneven cells
A grade like 4AB means:
4: well‑expanded blastocyst
A: strong inner cell mass
B: moderate trophectoderm
This grading represents a good quality embryo with reasonable implantation potential. However, grading is just one factor in IVF success and higher grading does not guarantee outcome.
This grading system is as per the ESHRE/ALPHA Istanbul Consensus 2025.
2. What’s the difference between cleavage‑stage and blastocyst‑stage embryos?
Embryos can be transferred or frozen at two main stages:
Cleavage‑stage embryos (Day 2–3)
Look like small clusters of cells (e.g., 6–8 cells)
Graded by cell number, symmetry, and fragmentation
They have not yet formed the structures seen in blastocysts
Blastocyst‑stage embryos (Day 5–6)
Have differentiated into the ICM and TE
Graded using the number‑letter system (e.g., 4AB)
More developmental information is available at this stage
Both stages can lead to healthy pregnancies. Clinics choose the stage based on the number and success of growing embryos and the patient history. Generally, clinics favour blastocyst embryos over cleavage, and according to the latest systematic review, blastocyst transfers do lead to slightly improved outcomes (Glujocsky et al, 2026)
3. Do lower‑graded embryos still lead to healthy pregnancies?
Yes — absolutely. Embryo grading is a snapshot, not a destiny.
The success rates of lower graded embryos are lower, and so higher grading embryos should always be transferred first. However, the success rate of not using a low-grade embryo is always 0%. So for many, still utilising lower grade embryos is important.
As evidenced by data from a Canadian Clinic, lower grade embryos should be considered for transfer as they can result in live births, however we must be honest with patients that their success rates are significantly lower than higher graded embryos (Lai, I et al, 2020)
The most important factor is whether the embryo is chromosomally normal, something grading cannot determine. Embryologists use grading to choose which embryo looks most likely to implant, but even “beautiful” embryos can fail, and “average” ones can succeed.
4. Why do embryos stop growing (“embryo arrest”), and is it my fault?
Embryo arrest means the embryo stops dividing or developing in the lab. This is common and usually due to factors outside anyone’s control.
Common reasons include:
· Chromosomal abnormalities — the most frequent cause, up to 94% (McCoy et al., 2023)
· Egg or sperm factors — often random, not related to health or lifestyle
· Natural selection — many embryos would not continue developing even in the body
· Laboratory conditions — although modern labs minimise this risk
Embryo arrest is not caused by anything you did or didn’t do. It is a natural part of human reproduction; even in natural conception, most embryos arrest before implantation. However, this doesn’t mean it can’t be really disheartening when it happens to your embryos.
5. Does a higher grade mean a higher chance of success?
Higher grades can help embryologists prioritise embryos, but they do not guarantee implantation or pregnancy.
What grading can tell you:
Which embryo looks structurally strongest today
What grading cannot tell you:
Whether the embryo is genetically normal
Whether it will implant
Whether it will become a baby
Many clinics emphasise that grading is not predictive — it is simply a way to describe appearance. Clinics that we work with, such as the Evewell we aim to transfer the highest-grade blastocyst on Day 5 of a fresh cycle as this is the most optimal time when your uterus is ready to receive an embryo (Whitney, 2022)
6. Why do different clinics use different grading systems?
There is no universal grading system. Some clinics use:
Numbers + letters (e.g., 4AB)
Modified Gardner grading
Clinic‑specific scales
Cleavage‑stage grading (cell number + fragmentation)
This can make reports look inconsistent, but the underlying principles are similar: describing development, ICM quality, and TE quality. If you are confused about anything in your grading report, call your clinic and ask to discuss with an embryologist.
Explore more about Fertility Support
At A D Acupuncture, we work hard to support patients navigating fertility and IVF. To learn more about how we can support you as you go through IVF, get in touch or book with us. We are available for face to face support with acupuncture in London, and working online to provide support across the UK.
References:
Glujovsky D, Cornelisse S, Blake D, Quinteiro Retamar AM, Alvarez Sedo CR, Ciapponi A. Blastocyst‐stage versus cleavage‐stage embryo transfer in assisted reproductive technology. Cochrane Database of Systematic Reviews 2026, Issue 1. Art. No.: CD002118. DOI: 10.1002/14651858.CD002118.pub7.
Lai, I., Neal, M., Gervais, N., Amin, S., Taerk, E., & Faghih, M. (2020). Transfers of lower quality embryos based on morphological appearance result in appreciable live birth rates: a Canadian center's experience. F&S reports, 1(3), 264–269. https://doi.org/10.1016/j.xfre.2020.09.003
McCoy RC, Summers MC, McCollin A, Ottolini CS, Ahuja K, Handyside AH. Meiotic and mitotic aneuploidies drive arrest of in vitro fertilized human preimplantation embryos. Genome Medicine 2023;15:77. https://doi.org/10.1186/s13073-023-01231-1.
The Working Group on the Update of the ESHRE/ALPHA Istanbul Consensus, Coticchio G, Ahlström A, Arroyo G, Balaban B, Campbell A, et al. The Istanbul consensus update: a revised ESHRE/ALPHA consensus on oocyte and embryo static and dynamic morphological assessment†,‡. Hum Reprod 2025;40:989–1035. https://doi.org/10.1093/humrep/deaf021.
Whitney, E “How The Evewell grades embryos” 2022 https://www.evewell.com/support/embryo-grading
