The embryo has the appropriate cell number for stage of development (eg., on day three, embryos should have 8 cells, also known as blastomeres) and the cells are even in their size and shape. The embryos should have little or no fragmentation (small amounts of extracellular debris) between cells.
The embryo has the correct development of cell numbers (6-8 cells on day three), with a small amount of fragmentation (20% fragmentation), or slight uneveness of blastomere sizes. grade 3 the embryo contains blastomeres that are uneven in size and has more than 30% fragmentation. embryos developing slowly will also receive a grade of 2 or 3 even if they are normal otherwise. grade 4 the embryo contains very few, if any, viable appearing cells and has a high percent fragmentation (over 50%).