Abstract: Objective: To explore the developmental potentiality of the embryos with low morphological score on Day 3(D3) during in vitro blastocyst culture, and analyze the clinical and embryonic factors affecting the blastocyst formation. Methods：Those embryos with D3 low score were further cultured to form blastula through the sequential methods. The blastocyst formation rate and the high-quality blastocyst formation rate were calculated. Those factors related to blastocyst formation were analyzed, including age，antral follicle count，bFSH， total dose of Gn，E2 level on hCG day，the number of retrieved oocytes，the number of blastomere on D3, and the embryonic fragmentation on D3. Results：A total of 1 745 low score embryos from 422 cycles were collected. 1 046 blastulas were formed（59.9%） after in vitro culture，among which 763（43.7%） were high-quality blastulas. There were no significant differences in the blastocyst formation rate and the high-quality blastocyst formation rate among subgroups grouping with the antral follicle count，bFSH， total dose of Gn，E2 levels on hCG day, or the number of retrieved oocytes（P＞0.05）. There was no significant difference in the blastocyst formation rates among subgroups grouping with age（P＞0.05），while there was significant difference in the high-quality blastocyst formation rate among those subgroups （P＜0.05）. Particularly, there were significant differences in the blastocyst formation rate and the high-quality blastocyst formation rate among subgroups grouping with the number of blastomere or the degree of embryonic fragmentation on D3（P＜0.05）. Conclusions：Embryos with low score have developmental potentiality on different degrees. The number of blastomere and the degree of embryonic fragmentation on D3 significantly influence the blastocyst formation of those embryos with low score. The factors of age, the number of blastomere and the degree of embryonic fragmentation significantly influence the high-quality blastocyst formation of embryos with low score.

Key words: Embryonic development, Blastula, Factor analysis, statistical, Cells, cultured