Blastocyst grading or quality is determined by evaluating the outer ring of cells, known as the trophectoderm or trophoblastic cells, that will eventually form the placenta; the inner cell mass or ICM, which is made up of the stem cells that the baby will develop from.
Scoring of blastocysts is an imperfect science, and some very nice-looking blastocysts do not necessarily produce a pregnancy. However, the basic rule of thumb is that the best embryos make it to the blastocyst stage, and therefore has a greater chance statistically of producing an ongoing pregnancy than a lesser quality one.
