Progesterone exposure duration affects live birth rates depending on blastocyst expansion stage in frozen embryo transfers.

This retrospective cohort study analyzed data from 2,058 women who underwent a single frozen–thawed day-6 blastocyst transfer. The primary exposure was the duration of progesterone exposure, comparing day-6 versus day-7 initiation. The primary outcome was live birth rate (LBR). Causality was not established; associations were evaluated via logistic regression models.

Overall, the live birth rate between the day-6 and day-7 progesterone groups was comparable, with a P value of 0.691. However, a highly significant interaction was observed between progesterone exposure duration and blastocyst expansion stage (P for interaction < 0.01). Safety data, including adverse events and tolerability, were not reported.

Among early-stage blastocysts (stages 3-4), live birth rates were significantly lower following day-7 versus day-6 progesterone exposure (adjusted OR = 0.70, 95% CI: 0.55-0.87; P = 0.002). Conversely, among late-stage blastocysts (stages 5-6), live birth rates were significantly higher following day-7 versus day-6 exposure (adjusted OR = 1.82, 95% CI: 1.34-2.48). The P value for this specific comparison was not reported.

Limitations include the retrospective nature of the study and the lack of reported safety outcomes. Practice relevance is uncertain as the study does not report funding or conflicts of interest, and clinical outcomes beyond live birth rate were not assessed. Clinicians should interpret these findings conservatively regarding progesterone timing strategies.