One of the best articles that I have read on the grading of embryos.
The Advanced Fertility Center of Chicago discusses how the embryo quality is assessed at day 3 stating, “There are many embryo grading systems that differ in how they assign grades and in whether a low number grade indicates the best or the worst embryo. We assess the “quality” of embryos from in vitro fertilization by carefully evaluating and scoring some aspects of their appearance.”
Things like cell number, cell regularity, degree of fragmentation and presence of multinucleation are all looked at during the assessment stage.
The below picture, from their article, shows a “perfect” looking 8-cell embryo (day 3 embryo). There is no embryo fragmentation and the cells are very even, regular, and similarly sized. We call this is a “high quality” day 3 embryo. The grading assigned to this embryo was 8 cell, grade 4.