How is embryo quality assessed?
The most widely used criteria for selecting the best embryos for transfer has been based on cell number and the appearance of the cells. Some programs identify early cleaving 2 cell embryos at 25 to 26 hours after insemination, because these may be more likely to lead to blastocysts and clinical pregnancies. By 42-44 hours after insemination, the best embryos have four or more cells and less than 20% fragmentation. Some embryos will have uneven or asymmetrical cells and some will have one or more cells that are disintegrating. Cellular fragments that result from this disintegration are only an indicator of quality when they are severe. By 72 hours, or Day 3 of development, the most favorable embryos consist of at least 6 cells, and an embryo with 7 or 8 cells is considered ideal. By Day 4, an embryo with high implantation potential should form a morula, and by Day 5, the top embryos form blastocysts. Blastocysts are graded by their degree of expansion and the appearance of the cells th
