Contrasting patterns in in vitro fertilization pregnancy rates among fresh autologous, fresh oocyte donor, and cryopreserved cycles with the use of day 5 or day 6 blastocysts may reflect differences in embryo-endometrium synchrony.

OBJECTIVE: To compare the effect of day 5 and day 6 blastocyst transfers on patterns of implantation rates and pregnancy rates (PRs) among fresh autologous, oocyte donor, and frozen ET (FET) cycles.
DESIGN: Retrospective study.
SETTING: Private fertility center. PATIENT(S): The study included 377 fresh autologous cycles, 106 autologous FET cycles, and 56 fresh oocyte donor cycles. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): Implantation rates and clinical PRs. RESULT(S): The clinical PR for day 5 blastocyst transfers was higher than for day 6 blastocyst transfers in fresh autologous cycles (PRs, 51.0% and 33.3%, respectively). However, there was no significant difference between transfers of blastocysts cryopreserved on day 5 and day 6 in FET cycles (PRs, 63.6% and 58.9%, respectively). Furthermore, day 6 blastocyst transfers significantly outperformed day 5 transfers in donor cycles (PRs, 63.0% and 86.2%, respectively), a reversal of the pattern seen in the fresh autologous cycles. Day 6 blastocysts were associated with a significantly greater PR in FET cycles than in fresh autologous cycles (58.9% and 33.3%, respectively). CONCLUSION(S): The superior PRs with day 5 blastocyst transfers in fresh autologous cycles and with day 6 blastocysts in donor cycles may have resulted from better synchrony with endometrial development. This was further supported by the superior performance of day 6 blastocysts in FET cycles relative to their fresh counterparts. Similar PRs with cryopreserved day 5 and day 6 blastocysts in FET cycles may reflect that, in these cycles, day 5 and day 6 blastocysts had equivalent quality and similar synchrony with the endometrium.