Association between leading follicle size and clinical pregnancy in gonadotropin-stimulated intrauterine insemination cycles with human chorionic gonadotropin trigger: a retrospective cohort study.

CPRs were 16.5% (55/334), 12.5% (48/385), and 6.1% (5/82) for leading follicle sizes of 15-16 mm, 17-18 mm, and 19-21 mm, respectively.

In the primary spline-based model, adjusted predicted clinical pregnancy rate peaked at approximately 16 mm and declined beyond 17 mm.

Compared with 16 mm, the adjusted risk ratio (aRR) was significantly lower at 18 mm (aRR 0.66, 95% CI 0.47-0.92), 19 mm (aRR 0.42, 95% CI 0.22-0.81), and 20 mm (aRR 0.27, 95% CI 0.10-0.75).

First-cycle-only spline model and generalized additive model sensitivity analyses supported a similar downward trend beyond approximately 17 mm.