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

Background Frozen embryo transfer (FET) has become a pivotal component of assisted reproductive technology (ART), particularly following the cancellation of fresh embryo transfer cycles. However, the optimal timing for FET after fresh cycle cancellation remains uncertain, with conflicting evidence on whether shorter or longer intervals yield better pregnancy outcomes. Existing studies often overlook age-specific effects, despite known differences in endometrial receptivity and ovarian reserve between younger and older women. This study investigates the impact of varying time intervals on FET outcomes, stratified by maternal age, to provide evidence-based guidance for clinical decision-making. Methods Retrospective analysis of women undergoing their first frozen embryo transfer after cancellation of fresh embryo transfer following In Vitro Fertilization / Intracytoplasmic Sperm Injection at a single center between January 2018 and December 2023. Participants were stratified by 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. Outcomes included live birth rates (LBRs), clinical pregnancy rates (CPRs), and pregnancy loss. Multivariable logistic regression adjusted for confounders. Results Among 1837 women, those < 35 years with longer intervals (3–6 and ≥ 6 months) showed a lower clinical pregnancy (AOR 0.77 [95% CI, 0.57–1.05], 0.69 [95% CI, 0.47-1.00]) and live birth rates (AOR 0.72 [95% CI, 0.54–0.95], 0.71 [95% CI, 0.50–1.02]). Women aged 35–40 years with intervals ≥ 6 months had a significantly increased risk of clinical pregnancy loss (AOR 2.11 [95% CI, 1.10–2.96]). Conclusions For women younger than 35 years, FET within three months after fresh cycle cancellation appears optimal. For women between 35 and 40 years, limiting the interval to six months may minimize clinical pregnancy loss.