Embryo grading is a standardized method used within In Vitro Fertilization (IVF) to visually assess the quality and developmental potential of an embryo. Embryologists examine the embryo’s morphology, or physical characteristics, under a microscope. The primary goal is to select the most viable candidate for transfer to the uterus, maximizing the chance of a successful pregnancy.
The grading system evaluates structural integrity and developmental pace, not genetic health. Embryologists score embryos at two distinct developmental windows, which helps prioritize embryos for immediate use or cryopreservation.
Timing and Milestones of Embryo Development
Embryos created through IVF are monitored and graded at specific points corresponding to major biological milestones. The two primary windows for assessment are the cleavage stage (Day 2 or 3) and the blastocyst stage (Day 5 or 6).
The cleavage stage occurs around Day 3, where the embryo is a cluster of individual cells called blastomeres. The embryo undergoes rapid cell division, and a Day 3 embryo should ideally have between six and ten cells. Quality assessment is based on the appearance of these blastomeres.
The blastocyst stage is reached on Day 5 or Day 6. The embryo has undergone significant differentiation and compaction, organizing cells into two distinct groups and forming a fluid-filled cavity. These groups are the Inner Cell Mass (ICM), which forms the fetus, and the Trophectoderm (TE), which develops into the placenta.
Grading the Cleavage Stage Embryo (Day 3)
Assessment of the Day 3 cleavage-stage embryo focuses on three main morphological parameters: cell number, fragmentation percentage, and cell symmetry.
The cell number is a straightforward metric; six to ten blastomeres indicates a normal growth rate. Embryos dividing too slowly or too quickly have a lower potential for further development.
The second parameter is fragmentation, which is non-nucleated cellular debris found outside the blastomeres. Excessive fragmentation is undesirable as it can compromise cellular function. Laboratories often grade fragmentation on a scale, such as 1 (little to no fragmentation) to 4 (significant debris).
The third observation involves the symmetry and uniformity of the blastomeres. Cells should be of similar size and shape, reflecting an even division process. Embryos with irregular cells are assigned a lower quality grade.
Grading the Blastocyst Stage Embryo (Day 5/6)
The grading of a blastocyst (Day 5 or Day 6) utilizes the internationally recognized Gardner grading system, which provides a detailed three-part score (e.g., “4AB”). The numerical component describes the degree of expansion, while the two letters assess the quality of the Inner Cell Mass (ICM) and Trophectoderm (TE).
Numerical Grade (Expansion)
The number (1 to 6) indicates the stage of blastocyst expansion and hatching status. Progression through these stages indicates developmental vigor.
- A score of 1 represents an early blastocyst where the fluid cavity is less than half the volume.
- A grade 3 is a full blastocyst where the cavity fills the entire volume.
- A grade 4 is an expanded blastocyst where the cavity is larger than the original embryo.
- A grade 5 is a hatching blastocyst, where the TE is beginning to emerge.
- A grade 6 is a fully hatched blastocyst that has completely escaped its shell.
First Letter Grade (Inner Cell Mass – ICM)
The first letter (A, B, or C) assesses the quality of the ICM, the cluster of cells that will form the fetus.
- Grade A indicates many tightly packed cells (highest quality).
- Grade B describes several cells that are more loosely grouped.
- Grade C is assigned to a cluster with very few cells.
Second Letter Grade (Trophectoderm – TE)
The second letter (A, B, or C) evaluates the quality of the TE, the outer layer of cells that will form the placenta.
- Grade A has many cells forming a cohesive, uniform layer.
- Grade B has fewer cells that form a looser, less organized layer.
- Grade C is characterized by very few cells.
The combination of these three scores provides a comprehensive picture of the blastocyst’s morphology. For instance, a 4AA indicates the highest quality ICM and TE in a fully expanded blastocyst. Letter grades are assigned only to blastocysts that have reached an expansion stage of 3 or higher, as the ICM and TE are not clearly defined earlier.
Interpreting the Grades for Transfer and Cryopreservation
Embryo grades guide embryologists in deciding which embryos to transfer and which to freeze. Highest-graded embryos, such as 5AA or 4AB blastocysts, are prioritized for immediate uterine transfer due to their superior morphological characteristics. Selecting the single best embryo for transfer is standard practice to reduce the risk of multiple pregnancies.
The grade determines suitability for cryopreservation (freezing). Embryos must meet quality thresholds to survive the freezing and thawing process. Cleavage-stage embryos selected for freezing usually have minimal fragmentation (less than 20%) and a normal cell number.
For blastocysts, those with an expansion grade of 3 or greater and at least B quality for both the ICM and TE are considered good candidates. While a higher grade correlates with a greater likelihood of implantation, the grade does not guarantee success, as genetic factors also play a significant role.