Background: Following preimplantation genetic testing for aneuploidy (PGT-A) cycles, the postimplantation development of vitrified-warmed blastocysts expanded on day 5 (D5) versus (vs.) day 6 (D6) has remained unclear, indicating that the ploidy status may not be the only intrinsic factor influencing the development of implanted embryos. Methods: Single-embryo transfers (SETs) of euploid or low-level mosaic blastocysts were performed between January 2018 and December 2020. Continuous TL monitoring was then conducted to evaluate the developmental features of individual embryos. After the data of successful SETs were analyzed, the LB rates were compared between the D5 (n = 200) and D6 (n =56) groups as primary outcomes. The correlations between the embryonic features and LB probability were then determined as secondary outcomes. Results: According to the univariate (odds ratio [OR] = 0.445, 95% confidence interval [CI] = 0.216–0.916; P < 0.05) and multivariate (adjusted OR = 0.385, 95% confidence interval = 0.184–0.805, P < 0.05) logistic regression analyses, the LB probability was negatively associated with the D6 group. The LB rate of the D5group was higher than the D6 group (86% vs. 73.2%; P < 0.05). TL monitoring also revealed that, compared with the D5 implanted blastocysts, the D6 implanted blastocysts exhibited comparable dysmorphisms except the multinucleation at the 4-cell stage (12.5% vs. 3.5%, P < 0.05) and direct uneven cleavage (3.6% vs. 0%, P < 0.05). Moreover, D6 implanted blastocysts had considerably slower developmental kinetics and poorer blastocyst morphologies. Logistic regression analysis confirmed that the LB rate was not associated with developmental kinetics or dysmorphisms but rather with blastocyst morphology (inner cell mass [ICM] grade ≤ C vs. ICM grade A, adjusted OR = 0.212, 95% confidence interval = 0.058–0.77, P < 0.05; trophectoderm [TE] grade ≤ C vs. TE grade A, adjusted OR = 0.182, 95% confidence interval = 0.049–0.672, P < 0.05). Conclusions: After successful implantation, D6 blastocysts have a considerably lower LB rate than D5 blastocysts. As determined by TL monitoring, the diminished blastocyst morphology is the primary reason underlying the decreased likelihood of LB.