Cryopreserved embryo transfers suggest that endometrial receptivity may contribute to reduced success rates of later developing embryos.

OBJECTIVE: To evaluate viability and implantation potential of cryopreserved blastocysts according to the day of blastocyst expansion and cryopreservation.
DESIGN: Retrospective study.
SETTING: Private ART center. PATIENT(S): Three hundred and seventy-five patients undergoing embryo transfer with cryopreserved blastocysts. INTERVENTION(S): Blastocyst cryopreservation on day 5, 6, or 7 after oocyte retrieval according to the day of blastocyst expansion and subsequent embryo transfer. MAIN OUTCOME MEASURE(S): Clinical pregnancy rate (PR) per embryo transfer. RESULT(S): Clinical PRs were similar between blastocysts cryopreserved on day 5 and blastocysts cryopreserved on day 6 (32% vs. 28%). The clinical PR was lower for blastocysts cryopreserved on day 7 (15%), but this difference was not statistically significant after accounting for the number of embryos per transfer (P=.15). CONCLUSION(S): Viability and implantation potential are similar for day 5 and day 6 blastocyst cryopreservation. Viability may be reduced for blastocysts cryopreserved on day 7, but not to the extent suggested by reports of fresh transfers. These results suggest that reduced success rates associated with fresh transfers of later developing blastocysts may be the result of asynchrony with endometrial receptivity instead of poorer embryo quality.