By Jayne Mullen, Scientific Director Victoria
When embryos reach the blastocyst stage on Day 5, embryologists use the Gardner grading system to assess how well each embryo has developed. This grading method is widely used in IVF laboratories around the world because it offers a structured way to understand an embryo’s potential for implantation and ongoing development.
Why Blastocyst Grading Matters
Not all embryos grow at the same pace or with the same quality. By the time an embryo becomes a blastocyst, its structure is developed enough for embryologists to examine key features. This gives clearer insight into which embryos are most suitable for transfer or freezing, supporting safe single‑embryo transfer and informed decision‑making throughout the IVF process
How the Gardner Grading System Works
Each blastocyst is given a three‑part score (for example, 4AA). The score is based on:
- Blastocyst Expansion (Number 1–6)
This number reflects how expanded or developed the blastocyst is:
- 1–2: Early blastocyst
- 3: Full blastocyst
- 4: Expanded blastocyst
- 5: Hatching blastocyst
- 6: Hatched blastocyst
Stages 3–6 are generally suitable for transfer or vitrification.
- Inner Cell Mass (ICM – First Letter)
The ICM becomes the foetus.
- A: Many tightly packed cells
- B: Several loosely grouped cells
- C: Very few cells
- Trophectoderm (TE – Second Letter)
The TE becomes the placenta.
- A: Many cells forming a cohesive layer
- B: Fewer, more loosely arranged cells
- C: Very few cells
What the Grades Mean
- 4AA, 5AA, 6AA: Excellent‑quality embryos with strong implantation potential
- AB, BA: Good‑quality embryos
- BB, BC: Moderate or lower quality
- C grades: Lower likelihood of implantation
It’s important to understand that grading assesses structure, not genetics. Many healthy babies are born from embryos with B‑grade components.
How Grading Helps Your IVF Journey
The Gardner system helps embryologists:
- identify the most promising embryo for transfer
- decide which embryos should be frozen for future use
- support safe single‑embryo transfer
- reduce the risk of multiple pregnancy
By providing a clearer picture of embryo development, this grading system plays a key role in tailoring IVF treatment and maximising the chance of success.
If you’d like to learn more about how embryos grow through each stage, you can read more here: The Stages of Embryo Development
