Embryo grading is a standardized visual assessment used by embryologists to evaluate the quality and developmental potential of embryos created during In Vitro Fertilization (IVF). This process is central to maximizing the success of an IVF cycle by identifying the most promising embryos for uterine transfer or cryopreservation. The grading system provides a prediction of an embryo’s likelihood to implant successfully, though it does not guarantee a pregnancy. Embryologists assign a score based on specific morphological criteria at different developmental stages to guide clinical decisions. This assessment is a snapshot of the embryo’s appearance and is not a measure of genetic health, which requires separate testing.
Grading the Cleavage Stage Embryo (Day 3)
The assessment of embryos on Day 3 post-fertilization focuses on the cleavage stage, where the embryo is rapidly dividing. The primary criterion evaluated is the number of cells, or blastomeres, present. Ideally, a Day 3 embryo should contain between six and ten cells, with eight cells often considered optimal for the developmental timeline. Deviations from this range, such as too few or too many cells, can suggest reduced viability.
The second major factor in Day 3 grading is the degree of fragmentation, which refers to small, anucleated pieces of cellular debris visible outside the main blastomeres. While fragmentation is common, a higher percentage is associated with a lower potential for successful development. Embryos with less than 10% fragmentation are considered high quality, whereas those with 25% or more are viewed as having low implantation potential.
Embryologists also assess the symmetry and uniformity of the blastomeres; cells of relatively equal size and shape indicate healthy, even division. Day 3 grades are often assigned using a simple numerical system, such as a 1 to 4 scale, where the lowest number indicates the best quality based on cell number, symmetry, and minimal fragmentation. This grading system is less standardized across clinics than the system used for later-stage embryos.
Criteria for Blastocyst Grading (Day 5/6)
By Day 5 or Day 6, the embryo develops into a blastocyst, a complex structure differentiated into two distinct cell types and a fluid-filled cavity. The Gardner grading system is the most widely used method to assess blastocyst quality, assigning a three-part score: a number followed by two letters. The number (1 to 6) describes the degree of blastocyst expansion and its progress toward hatching from the outer shell, called the zona pellucida.
Expansion Grade 1 is an early blastocyst with a small fluid-filled cavity, while Grade 3 is an expanded blastocyst with a noticeably larger cavity. A Grade 5 denotes a hatching blastocyst, where the embryo is emerging from the shell, and Grade 6 is fully hatched. An expansion grade of 3 or greater indicates the embryo is ready for transfer or freezing.
The first letter describes the quality of the Inner Cell Mass (ICM), the cluster of cells that will form the fetus. The ICM is graded A, B, or C, where Grade A signifies many tightly packed cells, and Grade C indicates very few or irregular cells. The second letter assesses the Trophectoderm (TE) quality, the outer layer of cells that will develop into the placenta and surrounding membranes. TE quality is also graded A, B, or C, with Grade A representing many cells forming a cohesive, organized layer.
Understanding the Final Embryo Score
The blastocyst grade combines all three morphological components into a single, alphanumeric score (e.g., 4AB or 5BB). The number indicates the expansion stage, while the first and second letters represent the ICM and TE quality, respectively. For example, a 4AA blastocyst is fully expanded (4) and has the highest quality for both the fetal-forming ICM (A) and the placenta-forming TE (A).
The final score informs the embryologist’s decision regarding transfer priority and cryopreservation viability. Embryos with higher-quality scores (A or B grades for both cell masses) are generally transferred first due to their higher probability of implantation. Transfers using excellent-grade blastocysts may have a pregnancy rate around 65%, compared to approximately 50% for average-grade embryos.
Embryos with lower scores, such as 3CC, may still be viable but are considered a lower priority for transfer or freezing. The grade is a predictive tool, and an embryo’s score is not static; a lower-grade embryo may continue to develop and improve its appearance later.