Blastocyst 5bb on day 6 is a better quality blastocyst. Blastocyst 5bb is an evaluation of the quality of the blastocyst. The numbers represent the size of the blastocyst cavity and whether the blastocyst is hatched or not, numbers 1 to 6 represent the level of development of the blastocyst, the higher the number the more mature the blastocyst is. 5 means the blastocyst is in incubation. The two letters represent the classification of the inner cell mass and the trophoblast cells, respectively, where a is the highest and c is the lowest, while bb indicates that the inner cell mass has a small number of cells and is loosely arranged, which is an intermediate classification, and that the trophoblast cells consist of a small number of cells and are loosely arranged, which is also an intermediate classification. Blastocyst 5bb on day 6 was one of the better quality blastocysts and had a higher pregnancy rate.