Impact of interval after fresh cycle cancellation on first frozen embryo transfer outcomes

Background: Frozen embryo transfer (FET) has become an integral component of assisted reproductive technology (ART), particularly after cancellation of fresh embryo transfer cycles. However, the optimal timing of FET after fresh cycle cancellation remains unclear, with conflicting evidence as to whether shorter or longer intervals are associated with better pregnancy outcomes. Previous studies have largely overlooked age-specific effects, despite established differences in endometrial receptivity and ovarian reserve between younger and older women. This study aimed to evaluate the association between the interval from oocyte retrieval to FET and reproductive outcomes after fresh cycle cancellation, stratified by maternal age, to provide evidence-based guidance for clinical decision-making. Methods: fertilization/intracytoplasmic sperm injection at a single center between January 2018 and December 2023. Participants were categorized according to the interval from oocyte retrieval to FET: ≤3 months (n = 1,011), 3-6 months (n = 529), and ≥6 months (n = 297). Subgroup analyses were performed by maternal age. The primary outcomes were live birth rate (LBR), clinical pregnancy rate (CPR), and clinical pregnancy loss. Multivariable logistic regression was used to adjust for potential confounders. Results: A total of 1,837 women were included. Among women aged