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

OBJECTIVE: To study the association between the leading follicle size on the day of human chorionic gonadotropin (hCG) trigger and clinical pregnancy in gonadotropin-stimulated intrauterine insemination (IUI) cycles. DESIGN: Retrospective cohort study. SETTING: University-affiliated assisted reproduction unit. SUBJECTS: Women with ovulatory dysfunction or unexplained infertility undergoing gonadotropin-stimulated, hCG-triggered IUI cycles. EXPOSURE: Ovarian stimulation with gonadotropins, hCG trigger, and IUI. MAIN OUTCOME MEASURES: Clinical pregnancy rate (CPR) was the primary outcome. The primary analysis modeled leading follicle size continuously using a natural cubic spline within a modified Poisson regression model with robust standard errors clustered by woman. Secondary analyses used categorical modified Poisson models, and sensitivity analyses included a spline model of first-cycle-only and an adjusted generalized additive model (GAM). RESULTS: A total of 516 women with 801 gonadotropin-stimulated IUI cycles between 01/2019 and 12/2025 were included. 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 CPR 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). Modified Poisson regression supported this pattern, showing lower adjusted CPR in the 19-21 mm group versus 15-16 mm (aRR 0.35, 95% CI 0.14-0.84) and for follicle size >17 mm versus ≤17 mm (aRR 0.62, 95% CI 0.40-0.97). First-cycle-only spline model and GAM sensitivity analyses supported a similar downward trend beyond approximately 17 mm. CONCLUSIONS: A leading follicle size of 15-16 mm appears to be the optimal range for clinical pregnancy in gonadotropin-stimulated, hCG-triggered IUI cycles, whereas leading follicles >17 mm were associated with a lower adjusted clinical pregnancy probability. These findings provide evidence-based guidance for optimizing gonadotropin-IUI treatments.