Comparison of differences in development potentials between frozen-thawed D5 and D6 blastocysts and their relationship with pregnancy outcomes

Whether there are differences in the pregnancy outcomes of blastocysts cryopreserved during different developmental stages remains under debate because the results among studies are inconsistent. We analyzed blastocyst quality and pregnancy outcomes by considering blastocyst euploidy and investigated the differences in the development potential between blastocysts of different developmental stages (frozen-thawed day 5 [D5] and day 6 [D6] cycles) and their relationship with clinical pregnancy outcomes.

In total, 1374 D5 and 255 D6 frozen-thawed blastocyst transfer cycles were retrospectively analyzed. Additionally, the chromosome euploidy and clinical pregnancy rates of 237 blastocysts from 50 pre-implantation genetic diagnosis (PGS) cycles were statistically analyzed. The corresponding euploidy rate and pregnancy outcomes of the D5 and D6 blastocyst transfers were also compared.

The clinical pregnancy rate (47.2 vs 40.0 %; P = 0.04) and implantation rate (34.2 vs 28.8 %; P = 0.03) of the D5 blastocysts were higher than were those of the D6 blastocysts. However, the clinical pregnancy rate (52.4 vs 52.6 %; P = 0.97) and implantation rate (38.9 vs 35.6 %; P = 0.39) of the high-quality D5 blastocysts did not significantly differ from those of the high-quality D6 blastocysts. Analysis of blastocyst euploidy in 237 blastocysts examined in 50 PGS cycles showed that the euploidy rates of the D5 and D6 blastocysts were both 48.1 % (P = 0.99). The clinical pregnancy rate of the D5 blastocysts (48.5 vs 17.6 %; P = 0.03) was higher than that of the D6 blastocysts. The euploidy rates (55.2 vs 55.3 %; P = 0.99) and clinical pregnancy rates (60.0 vs 42.9 %; P = 0.77) of the high-quality D5 and D6 blastocysts did not differ. The euploidy rate (55.3 vs 41.5 %, P = 0.03) and clinical pregnancy rate (54.5 vs 25.0 %, P = 0.03) of the high-quality blastocysts were higher than were those of the poor-quality blastocysts.

The euploidy rates between the D5 and D6 blastocysts did not differ. High-quality D6 blastocysts in frozen-thawed cycles had similar developmental potential and pregnancy outcomes compared to those of high-quality D5 blastocysts. The quality of the blastocysts was an important factor that affected the pregnancy outcomes of the frozen-thawed cycles.