Embryo selection
Embryo selection is based on several factors, including how the embryo is growing, the stage of development it has reached, the appearance and organisation of the cells, and any additional information available from time-lapse monitoring or genetic testing, where these are part of treatment.
Embryo grading is an important part of this process. It helps us prioritise embryos, but it cannot predict with certainty whether an embryo will implant or lead to a pregnancy. Higher-grade embryos are generally associated with a greater chance of implantation and pregnancy, but lower-grade embryos can still result in healthy pregnancies and live births.
When does embryo transfer occur?
Embryos are usually transferred once they have reached the blastocyst stage, typically around five days after fertilisation. A blastocyst contains hundreds of cells and has begun to organise into the cell groups that will eventually form the baby and placenta.
Allowing embryos to develop to the blastocyst stage gives embryologists more information about embryo development and helps guide which embryo is selected for transfer.
In some circumstances, an embryo transfer may be performed earlier, around Day 3 of development. This may be considered when there are fewer embryos available or when the clinical and laboratory team consider it the most appropriate option for an individual treatment cycle.
Your fertility specialist and embryology team will recommend the most appropriate transfer timing based on your individual treatment plan and the number and development of available embryos.
Understanding embryo grading:
Embryo grading is a visual assessment of how an embryo has developed in the laboratory. The type of grade used depends on the stage of embryo development.
Embryos are graded differently depending on whether they are assessed at the cleavage stage, around Day 3, or at the blastocyst stage, usually around Day 5 or 6.
- Day 3 embryos are graded by the number of cells and the amount of fragmentation seen.
- Blastocysts are graded by how expanded the embryo is and the appearance of two important cell groups.
Grading helps us decide whether an embryo is suitable for transfer or freezing, and how embryos should be prioritised if more than one embryo is available. It is one part of the overall decision-making process, alongside clinical information and, where applicable, time-lapse assessment or genetic testing results.
How to read embryo grades
Embryos are graded differently depending on whether they are assessed at the cleavage stage, around Day 3, or at the blastocyst stage, usually around Day 5 or 6.
Day 3 cleavage-stage embryo grades
A Day 3 embryo grade is written as the number of cells followed by a letter describing fragmentation. For example, 8A describes an embryo with eight cells and less than 10% fragmentation. A Day 3 embryo with a minimum grade of 6C is generally considered suitable for transfer. Embryos below this level may only be considered after fertility specialist approval and discussion with the patient.
| Day 3 grade | What it describes | Explanation |
| Cell number | How many cells are visible | By Day 3, embryos are commonly assessed by their cell number. An embryo with around eight cells and even cell size is considered to be developing as expected. |
| A | Less than 10% fragmentation | Very little fragmentation is visible. |
| B | 10-25% fragmentation | Some fragmentation is visible. |
| C | More than 25% fragmentation | More fragmentation is visible. |
Embryologists may also consider other features, such as cell symmetry, multinucleation, direct or reverse cleavage, and the timing of development. These features help with embryo ranking but do not automatically mean an embryo cannot be used.
Blastocyst grades
A blastocyst grade contains a number and two letters. For example, 4AB describes an expanded blastocyst. The first letter, A, describes the inner cell mass, and the second letter, B, describes the trophectoderm.
| Blastocyst grade | What it describes | Explanation |
| Expansion stage (1-6) | How developed and expanded the blastocyst is | A higher number generally means the blastocyst has expanded further. |
| ICM grade A | Inner cell mass | Many well-organised cells in the part of the embryo that develops into the baby. |
| ICM grade B | Inner cell mass | A moderate number of cells in the part of the embryo that develops into the baby. |
| ICM grade C | Inner cell mass | Fewer cells are visible in the part of the embryo that develops into the baby. |
| TE grade A | Trophectoderm | Many well-organised cells in the part of the embryo that develops into the placenta. |
| TE grade B | Trophectoderm | A moderate number of cells in the part of the embryo that develops into the placenta. |
| TE grade C | Trophectoderm | Fewer cells are visible in the part of the embryo that develops into the placenta. |
A higher grade generally suggests a greater chance of implantation and pregnancy, but it is not a guarantee. Embryos with lower grades may still be suitable for transfer or freezing and can still result in healthy babies.
Additional information used in embryo selection
Depending on the type of treatment, embryologists may have additional information to support embryo selection.
- Time-lapse monitoring may provide information about how an embryo has developed over time.
- AI-based decision support tools may help compare embryos with similar grades when time-lapse information is available.
- Genetic testing results may be used when preimplantation genetic testing is part of the treatment plan.
These tools support embryo selection decisions but do not replace embryologist assessment or clinical decision-making. No test or grading system can predict pregnancy with certainty.
