Embryo grading: what the letters and numbers mean
Learn how cleavage-stage and blastocyst grades are assigned, why laboratories can differ and what a grade cannot promise.
Embryo grading is a shorthand description of morphological features an embryologist sees under the microscope at a specific time. At cleavage stage it may include cell number, similarity in size and fragmentation. At blastocyst stage, expansion, the inner cell mass and trophectoderm may be assessed. A system helps standardise reporting and relative selection.
A grade is not a label for a “good baby” or “bad baby”. It describes appearance but cannot show every chromosome, gene activity or what will occur inside the uterus. A high grade cannot guarantee pregnancy, while a lower grade does not automatically remove the possibility of a healthy birth.
What is assessed at cleavage stage?
On days two and three, an embryologist records cell number in relation to the observation hour and whether cells appear relatively even in size. The proportion of small anuclear fragments, multinucleation and cytoplasmic features may also be part of the protocol. Laboratories do not all use an identical scale.
A number from one clinic should therefore not be compared directly with a report from another. Each laboratory has its observation schedule, culture system and validated terminology. Ask which day and grading system were used. The same embryo’s description can change as development continues.
- Cell count appropriate to observation time
- Cell symmetry and degree of fragmentation
- Multinucleation and other morphological notes
How to read a blastocyst code
A blastocyst grade commonly begins with a number for expansion, followed by two letters. The first letter describes the appearance of the inner cell mass, which contributes to the fetus, and the second describes the trophectoderm, which contributes to the placenta. Exact definitions depend on the selected system.
Choosing between two codes is not simply alphabetical. The day the embryo reached blastocyst, its expansion, condition after warming and the patient’s clinical history may be relevant. An embryologist should translate the code into plain language and explain its relative position among the embryos available.
- A number for blastocyst expansion
- First letter for the inner cell mass
- Second letter for the trophectoderm
How grades should and should not guide decisions
Grade is one item used to prioritise embryos for transfer or freezing. Age, previous treatment, day of development, PGT results when available and uterine factors are considered separately. A clinical team should not present a grade as an individual live-birth percentage.
Decisions not to freeze or to discard a lower-graded embryo should follow validated laboratory criteria and informed consent. Understand that “best” usually means a relative choice within the embryos available in that cycle. You can ask why each remaining embryo was frozen, observed further or excluded.
- Combine grade with other clinical information
- Do not treat it as a live-birth guarantee
- Know freezing and discard criteria before treatment
Frequently asked questions
Does an AA embryo guarantee a healthy baby?
No. The code describes blastocyst appearance. It cannot guarantee chromosome status, implantation, continuation of pregnancy or the health of a child.
Can a lower-grade embryo be transferred?
Sometimes. Suitability depends on laboratory criteria, development, available alternatives and clinical history. Your team should provide individual advice.
Ask an Övlad Payı embryologist to translate each embryo grading code using development day, expansion and cell groups. Use the code as relative laboratory information that supports a decision, not as a prediction.
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This article is for patient education and does not replace individual medical advice.
