Small follicle diameter before ovulation may not compromise live birth following the natural frozen embryo transfer cycle.

Objective: Assessing the association between follicle diameter and live birth rates (LBRs) following natural frozen embryo transfer (FET) cycles to justify cycle cancellations.

Methods: A retrospective study was performed on natural FET cycles. Cycles (n = 7745) were categorized according to their preovulatory follicle size: 3804 in the control group (18-22.9 mm), 894 in group 1 (13.5-15.9 mm), 2546 in group 2 (16-17.9 mm), and 501 in group 3 (23-35.5 mm). A multivariate generalized estimating equation and a generalized additive model were used to evaluate the association between leading follicle size and LBR when follicle size was considered a categorized or continuous variable, respectively. In addition, propensity score (PS) matching analysis was used to evaluate the effect of extreme follicle sizes (<15 mm, n = 410; >25 mm, n = 267).

Results: The LBRs were 45.8%, 43.6%, 45.8%, and 44.3% in the control group and groups 1-3, respectively. The risk ratios (RRs) in comparison with the control groups in groups 1-3 were 0.95 (95% confidence interval [CI] 0.88-1.03), 1 (95% CI 0.95-1.06), and 0.97 (95% CI 0.87-1.07), respectively. No difference was observed between small extreme (<15 mm) and normal (18-23 mm) leading follicle size groups before and after PS matching. When large extreme (>25 mm) and normal (18-23 mm) leading follicle size groups were compared, the LBRs were 48.3% versus 45.8% (P = 0.462) before and 48.3% versus 49.8% (P = 0.795) after PS matching.

Conclusion: Intense monitoring in natural FET cycles might be unnecessary, and a more patient-friendly treatment manner could be considered.