Frozen blastocysts: Assessing the importance of day 5/day 6 blastocysts or blastocyst quality.

The aim of the present study was to analyze the high-quality blastocyst (HB) rate in all embryo frozen cycles and investigate the pregnancy outcomes for day 5/day 6 (D5/D6) blastocysts with respect to the blastocyst quality in programmed single vitrified-warmed blastocyst transfer (SVBT). We performed a retrospective study comparing D5/D6 HBs in in vitro fertilization/intracytoplasmic sperm injection (IVF/ICSI) for all blastocyst frozen cycles. Patients were <35 years at the oocyte collection in their first fresh cycle without fresh transfer. A total of 1,560 IVF/ICSI cycles and 5,328 blastocysts were analyzed. The IVF HB rate was higher than that of ICSI (52.7% vs. 42.6%; P<0.05). The D5 HB rate was much higher than the D6 HB rate (61.6% vs. 29.4%; P<0.05). There were 22.4% (349/1,560) cycles that only had D6 blastocysts, of which IVF cycles were lower than ICSI (19.8% vs. 28.5%; P<0.05). The clinical pregnancy rate and implantation rate in the D5 group were significantly higher than these rates in the D6 group (57.4% vs. 46.2%, 58.9% vs. 47.3%; P<0.05). However, the clinical pregnancy rate and implantation rate of the D5 HBs were not significantly different from those of the D6 HBs (60% vs. 54.5%, 62% vs. 56.3%; P>0.05). In conclusion, the fertilization method (IVF/ICSI) directly influences the HB rate and blastocyst development rates. When we controlled for patient age, transfer frequency, and endometrium on day 5, it was not the development stage (D5/D6), rather the transfer blastocyst quality that played an important role in pregnancy outcomes.