Abstract: Objective：To compare the clinical outcomes of single poor-quality blastocysts between D5 and D6 vitrified-warmed blastocyst transfer cycles, and to optimize the single transfer strategy of poor-quality blastocyst. Methods：A retrospective analysis of the clinical data from patients underwent vitrified-warmed poor-quality single-blastocyst transfer cycles in our center from January 2013 to December 2016. According to the quality of blastocyst and the day of blastocyst development, those patients underwent ET were divided into 4 groups as follows, the D5 including AC/BC group (Inner cell mass and trophoblast score were AC or BC, 53 cycles in total), the D5 including CA/CB group (51 cycles) and the D6 including AC/BC group (65 cycles), the D6 including CA/CB group (61 cycles). The clinical pregnancy rate and early abortion rate were compared. Results：The clinical pregnancy rate of D5 group was higher than that of D6 group (P＜0.05), and the clinical pregnancy rate of CA/CB (D5) group was statistically significantly higher than that of AC/BC (D6) group (P＜0.05). The early abortion rate of AC/BC group was statistically significantly higher than CA/CB group (P＜0.05). Conclusions：The D5 blastocysts with high trophoblastic ectoderm grade can be selected for transfer so as to improve the pregnancy rate and reduce the early abortion rate.

Key words: Embryo transfer, Cryopreservation, Pregnancy rate, Abortion, spontaneous