Association Between Pregnancy Outcomes and the Time of Progesterone Exposure of D6 Single-Blastocyst Transfer in Frozen-Thawed Cycles: A Retrospective Cohort Study.

Purpose: The objective of this study was to assess reproductive outcomes of D6 blastocysts transferred on day 6 in comparison to those transferred on day 7 of progesterone exposure in frozen-thawed embryo transfer cycles.

Patients And Methods: This retrospective cohort study included 2029 D6 single blastocysts from the first frozen-thawed embryo transfer cycles of patients at the Hospital for Reproductive Medicine Affiliated to Shandong University from February 2017 to January 2020. Participants were divided into Group A (blastocyst transferred on the 6th day of progesterone exposure, n=1634) and Group B (blastocyst transferred on the 7th day of progesterone exposure, n=395).

Results: The live birth rate was comparable between Group A and Group B (38.7% versus 38.7%, P=0.999). Subgroup analysis revealed a significantly higher preterm birth rate in D6 single blastocysts transferred on the 7th day than in those transferred on the 6th day of progesterone exposure for natural cycle frozen-thawed embryo transfer (5.2% versus 11.3%, P=0.020). After adjustment for potential confounders, the differences in the preterm birth rate in natural cycles persisted (adjusted odds ratio 2.347, 95% confidence interval 1.129-4.877, P=0.022).

Conclusion: In frozen-thawed embryo transfer cycles, transferring on the 6th or 7th day of progesterone exposure of D6 blastocysts did not affect the live birth rate; however, when a natural cycle protocol is adopted, the possible preterm risk of transferring D6 blastocysts on the 7th day of progesterone exposure should be noted.