Relationship Between Dominant Follicle Diameter on the hCG Trigger Day and Live Birth After Modified Natural Cycle Frozen Embryo Transfer: A Retrospective Cohort Study.

BackgroundThe impact of dominant follicular diameter at hCG trigger on live birth rate (LBR) in modified natural cycle frozen-thawed embryo transfer (mNC-FET) remains unclear. This study aimed to evaluate this relationship and explore whether flexible triggering based on follicle size is acceptable.MethodsA retrospective study included 1669 mNC-FET cycles. The cycles were divided according to the dominant follicle diameter on the hCG day: ≥16 mm (n = 1439) and <16 mm (n = 230). The LBR was compared between the groups, and multivariate logistic regression was performed. A subgroup analysis stratified by maternal age (≥ 35 years) was conducted.ResultsOverall, the LBR did not significantly differ between the <16 mm groups (49.0% vs 43.9%, P = 0.152; adjusted OR = 0.878, 95% CI 0.626-1.230). Among advanced-age women (≥35 years), the ≥16 mm group showed a significantly higher LBR (40.2% vs 28.0%, P = 0.048), but multivariate analysis revealed no independent association after adjusting for confounders (aOR = 0.963, 95% CI 0.517-1.791, P = 0.904).ConclusionDominant follicular diameter on the hCG day was not independently associated with the live birth rate in mNC-FET. The crude difference seen in women aged advanced-age women (≥35 yeaadjustment and should be viewed as hypothesis-generating rather than effect modification. Flexible triggering may be acceptable, while maternal age remains a key clinical consideration in individualizing trigger timing.