Interaction between progesterone exposure duration and blastocyst expansion stage on live birth following frozen–thawed transfer of day-6 blastocysts

Objective To determine the optimal duration of progesterone exposure before frozen–thawed embryo transfer (FET) of day-6 blastocysts. Methods This retrospective cohort study included 2,058 women who underwent single frozen–thawed day-6 blastocyst transfer between August 2021 and August 2024. Univariable and multivariable logistic regression models were applied to evaluate the association between the duration of progesterone exposure and live birth rate (LBR). Subgroup analyzes with formal interaction testing were conducted to assess potential effect modification by blastocyst expansion stage and other covariates. Results Overall LBR was comparable between the day-6 and day-7 progesterone groups (42.8% vs. 41.9%; P = 0.691). Crucially, a highly significant interaction was observed between progesterone exposure duration and blastocyst expansion stage ( P for interaction < 0.01). Among early-stage blastocysts (stages 3-4), LBR was significantly lower following day-7 versus day-6 progesterone exposure (37.3% vs. 46.2%; adjusted OR = 0.70, 95% CI: 0.55-0.87, P = 0.002). Conversely, among late-stage blastocysts (stages 5-6), LBR was significantly higher following day-7 versus day-6 exposure (50.2% vs. 36.5%; adjusted OR = 1.82, 95% CI: 1.34-2.48, P < 0.001). Conclusion While overall LBRs were similar between day-6 and day-7 progesterone regimens for day-6 blastocyst, optimal progesterone duration differed by blastocyst expansion stage: early-stage blastocysts benefited from day-6 exposure, whereas late-stage blastocysts derived benefit from day-7 exposure.