Twenty-two to 25 hours after fertilization, the zygote undergoes its first mitotic division (cleavage) and becomes a 2-cell embryo. A normally growing embryo is then expected to have 3-4 cells on the 2nd day (42-44 hours), 6-8 cells on the 3rd day (66-68 hours) and more than 10 cells on the 4th day.
The cell number is the first parameter that will define the grade of the embryo. Thus, if the number of blastomeres is unexpectedly low considering its developmental day, the embryo is evaluated as slow-growing. Additionally, cleavage-stage embryos are checked for the parameters below:
- Size and shape of blastomeres,
- Degree of fragmentation between blastomeres,
- Nucleus number in each blastomere,
- Cytoplasmic appearance,
- Signs of compaction.
Grade 1: Embryos with equal-sized blastomeres, including 0-10% fragmentation and with no granular structure in the cytoplasm are ranked as top quality;
Grade 2: Embryos with slightly unequal-sized blastomeres or including 0-10% fragmentation;
Grade 3: Embryos with either slightly unequal-sized blastomeres including 10-20% fragmentation or with significantly unequal-sized blastomeres including 20-25% fragmentation;
Grade 4: Blastomeres cannot be counted or are distinctly different from each other with a fragmentation more than >25%.
Intuitively, when the quality of the embryo is reduced, its potential to pursue its development, to sustain its vitality and to implant decreases.
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