Blastocyst Grading Basics
By the blastocyst stage, cells have differentiated into two distinct cell lines. One cell line is the inner cell mass (ICM), and this inner cluster of cells develops into the fetus. The other cell line is the trophectoderm epithelium (TE), and this outer sheet of cells forms the extra-embryonic components which develops into the placenta.
- The Gardner system consists of a number indicating the degree of blastocyst expansion:
1 = early blastocyst, blastocoel <50% volume;
2 = blastocyst, blastocoel >50% volume;
3 = full blastocyst, 100% blastocoel expansion;
4 = expanded blastocyst;
5 = hatching blastocyst;
6 = hatched blastocyst.
- The number indicating blastocyst explanation is followed by a letter grade designating the quantity and quality of the ICM:
A = many cells tightly compacted;
B = several cells loosely adhered;
C = very few cells;
D = no cells or degenerate or necrotic cells.
- The second letter grade designating the quantity and quality of the TE:
A = continuous layer of small identical cells;
B = noncontinuous layer with fewer cells;
C = noncontinuous layer with few small cells and large cells;
D = sparse distribution of large or flat or degenerate cells).
Thus, there are nine combinations of embryo grades for blastocysts that are not degenerated: AA, AB, BA, BB, AC, CA, BC, CB, and CC.
The goal of the scoring system is to aid in the selection of embryos with high implantation potential, to improve reproductive outcomes. Higher-grade embryos are more likely euploid, and although some embryo scores are well accepted as having implantation potential (e.g., AA is the best, and AB is better than AC)