Impact of interval between freeze-all retrieval and first frozen embryo transfer: a large retrospective cohort study

The optimal timing of frozen embryo transfer (FET) following freeze-all cycles remains uncertain, with limited and conflicting evidence. This retrospective cohort study evaluated whether the interval between oocyte retrieval and the first FET influences pregnancy outcomes in 2,229 first FET cycles performed between 2003 and 2023. The primary outcome was live birth rate (LBR) across four inter-cycle intervals (≤30, 31–60, 61–90, and >90 days), while the secondary outcome was clinical pregnancy rate (CPR). Although CPR and LBR declined with longer intervals in unadjusted analyses (p = 0.002 and 0.015, respectively), these associations were no longer significant after adjustment for maternal age, FET protocol, number of oocytes retrieved, embryo quality, and embryo stage. Propensity score matching yielded consistent results. Subgroup analyses further demonstrated no significant association between FET timing and LBR among normal, hyper-, or poor responders, or between natural-cycle or hormone replacement therapy FET cycles. Neonatal outcomes, including gestational age, preterm birth rate, and birth weight were comparable among groups. These findings suggest that delaying the first FET after a freeze-all cycle does not significantly influence live birth or neonatal outcomes, supporting flexibility in scheduling embryo transfer.