If you're going through IVF, you've likely heard your clinic mention embryo grading. Perhaps you've been told your embryos are "grade A" or "3BB" and wondered what these letters and numbers actually mean for your chances of success.
Understanding embryo grading can feel like decoding a secret language, but it doesn't have to be confusing. Let's demystify the science behind embryo grading and what it means in IVF.
What is Embryo Grading?
Embryo grading is the method embryologists use to assess the quality of embryos developing in the laboratory.
After fertilisation, your embryos are carefully monitored and evaluated based on their appearance and development. The goal is to identify which embryos have the best potential for implantation and a healthy pregnancy.
Embryo grading is a key part of fertility treatments and is used to assess human embryos created during IVF.
Embryos are graded at specific points in their development, such as day 3 and day 5/6, to help select the best candidates for transfer. However, it's crucial to understand that grading is based on what embryologists can see under a microscope—it's an assessment of appearance and development patterns, not a guarantee of success.
How Are Embryos Graded During IVF?
Embryos can be graded at different stages of development, and the grading system varies depending on when the assessment takes place. Different embryo grading systems are used to assess embryo quality at specific developmental stages, such as day 3 and day 5, and these systems help guide decisions about which embryos to transfer or freeze.
Embryologists often use an embryo grading chart to categorise embryos based on these systems.
Day 2-3 Grading (Cleavage Stage)
In the early days after fertilisation, grading is performed on the cleavage stage embryo, and embryos are assessed based on:
- Number of cells: By day 3, cleavage stage embryos typically have 6-10 cells
- Cell symmetry: How evenly the cells are divided
- Fragmentation: The amount of cellular debris present
The grading process begins after fertilisation and does not include the unfertilised egg.
Clinics often use a simple grading system like Grade 1-4 or Grade A-D, where Grade 1 or A represents the highest quality (even cell division, minimal fragmentation). During this stage, cell mass quality is also assessed to help determine embryo viability. Grade 4 or D indicates lower quality (uneven cells, significant fragmentation).
Day 5-6 Grading (Blastocyst Stage)
By day 5 or 6, blastocyst embryos are assessed following blastocyst formation. At this stage, embryos that continue developing reach the blastocyst stage. This is a more complex structure with a fluid-filled cavity. Blastocyst grading is more detailed and typically uses a three-part system:
Number (1-6) indicating expansion:
- 1-2: Early blastocyst (just starting to expand)
- 3: Cavity completely fills the embryo
- 4-5: Full blastocyst (blastocoel cavity fills the embryo), with grade 5 indicating a hatching blastocyst as the zona pellucida begins to thin or rupture
- 6: Hatched blastocyst (completely emerged from the zona pellucida)
Letter for Inner Cell Mass (ICM)– the part that becomes the...
