Embryo grading is a visual evaluation method used by embryologists during IVF to assess the developmental potential of embryos.
It helps select the best embryo(s) for transfer, freezing, or biopsy. Embryo grading is essential in IVF, but it should be viewed as one piece of the puzzle. It helps embryologists choose the best embryo for transfer, but genetic testing and uterine health are just as important in determining final outcomes.
Grading is a way to estimate an embryo’s likelihood of successful implantation based on appearance and developmental progress.
WHEN ARE EMBRYOS GRADED?
DAY 3 (CLEAVAGE STAGE)
Embryos are assessed based on:
-
Number of cells: Ideal = 6–8 cells
-
Cell symmetry: Evenly sized cells are preferred
-
Fragmentation: Less than 20–25% fragmentation (cellular debris) is considered ideal
At this stage, it’s still early and difficult to predict which embryos will progress well. However, some clinics still use Day 3 transfers when necessary.
DAY 5–7 (BLASTOCYST STAGE)
By Day 5, embryos that continue developing become blastocysts, which have more distinct structures:
-
Blastocoel: The fluid-filled cavity
-
Inner Cell Mass (ICM): Becomes the fetus
-
Trophectoderm: Becomes the placenta
Grading uses the Gardner system, which scores: -
Expansion stage (1 to 6) – how fully the embryo has expanded
-
ICM quality (A–C) – A is best
-
Trophectoderm quality (A–C) – A is best
Example: 4AA is an expanded blastocyst with top-quality ICM and trophectoderm a strong candidate for implantation.
HOW DOES EMBRYO GRADE AFFECT IVF SUCCESS?
Higher-graded embryos have a greater chance of:
-
Implantation
-
Leading to a clinical pregnancy
-
Resulting in a live birth
However, it’s not a guarantee. Some lower-graded embryos still result in healthy babies, and some highly graded ones may not implant.
Embryo grading is not diagnostic it’s an estimate based on appearance. It does not detect chromosomal abnormalities.
GRADING VS. GENETIC TESTING (PGT-A)
Preimplantation Genetic Testing for Aneuploidy (PGT-A) can identify embryos with the correct number of chromosomes (euploid) vs. abnormal (aneuploid). A highly graded but genetically abnormal embryo may look perfect but will fail to implant or result in miscarriage. A less visually ideal but chromosomally normal embryo may result in a healthy pregnancy. Therefore, combining embryo grading with PGT-A offers the most reliable selection process.
WHY BLASTOCYST (DAY 5–6) TRANSFER IS PREFERRED
By Day 5, weaker embryos often stop growing (natural selection). Surviving embryos have proven developmental strength. Morphological features are easier to grade at this stage. Blastocyst transfer allows for single embryo transfer (SET), reducing the risk of multiples without reducing success.
LIMITATIONS OF EMBRYO GRADING
-
It’s subjective two labs might grade the same embryo slightly differently.
-
It doesn’t account for uterine factors like:
-
Endometrial receptivity
-
Hormonal environment
-
Immune response
-
-
It cannot replace genetic screening (PGT), which provides direct insights into chromosomal health.
THE ROLE OF THE UTERUS IN IMPLANTATION
Implantation success is not just about the embryo the uterus must be receptive, and timing must be synchronised. Even the best embryo cannot implant if the uterine lining isn’t ready.
Read more about embryo grading in this article by Illume Fertility.
