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Objective. To assess the effect of embryonic age on the pregnancy outcomes of blastocyst graded as CC in single frozen-thawed blastocyst transfer cycles. Methods. A total of 565 patients undergoing single frozen-thawed blastocyst transfer at Shanghai TOWAKO Hospital from April 2017 to December 2019 were included in this study, and they were divided into high-quality blastocyst group (grade AA, AB, and BA; n=189), medium-quality blastocyst group (grade BB, AC, and CA; n=114), low-quality blastocyst group (grade CB and BC; n=140) and CC grade blastocyst group (n=122). And the CC grade blastocysts were further divided into CC grade blastocysts of Day 5 (D5 subgroup, n=26), Day 6 (D6 subgroup, n=81) and Day 7 (D7 subgroup, n=15) according to the embryonic age. Thereafter, the cycle characteristics and pregnancy outcomes of the four groups and the three embryonic age subgroups of CC grade blastocysts were compared. Results. The clinical pregnancy rate (58.7% vs 54.4% vs 47.1% vs 25.4%, P=0.000) and live birth rate (46.2% vs 43.2% vs 36.2% vs 17.6%, P=0.000) of the high-quality, medium-quality, low-quality, and CC grade blastocyst groups decreased sequentially. Multivariate analysis showed that embryo quality was significantly correlated with clinical pregnancy rate. Compared with high-quality blastocyst group, CC grade blastocyst group had a lower clinical pregnancy rate (aOR=0.81, 95%CI 0.71 - 0.93, P=0.003). The analysis of CC grade blastocyst embryonic age subgroups showed that the clinical pregnancy rate (42.3% vs 23.5% vs 6.7%, P=0.040) and live birth rate (31.6% vs 15.6% vs 0, P=0.133) of the D5, D6 and D7 subgroups also decreased sequentially. Multivariate analysis showed that the embryonic age had a significant impact on the clinical pregnancy rate of CC grade blastocyst transfer. Compared with the D5 subgroup, the D7 subgroup had a lower clinical pregnancy rate (aOR= 0.69, 95%CI 0.49-0.96, P=0.032). Conclusion. D5 CC grade blastocysts predict good pregnancy outcome. Although the clinical pregnancy rate of D6 CC grade blastocysts is low, they are still useable. However, D7 CC grade blastocysts are of little clinical value. [ABSTRACT FROM AUTHOR]