Dominant Follicle Size as a Predictor of Pregnancy in Letrozole Intrauterine Insemination.

Background/Objectives: Letrozole combined with intrauterine insemination (IUI) is a common first-line treatment for unexplained and anovulatory infertility; however, the optimal dominant follicle size at human chorionic gonadotrophin (hCG) triggering remains uncertain. We evaluated associations of follicle size with biochemical pregnancy, clinical pregnancy, and live birth. Methods: This retrospective cohort included 596 letrozole-IUI cycles from 454 women aged <40 years treated between May 2024 and August 2025. Cycles were grouped by dominant follicle diameter at trigger (17.0-18.9, 19.0-21.0, and >21.0 mm). Mixed-effects logistic regression accounted for repeated cycles. Results: Biochemical pregnancy, clinical pregnancy, and live-birth rates differed across groups and were highest at 19.0-21.0 mm; live-birth rates were 3.4%, 15.1%, and 8.6%, respectively (p = 0.008). Compared with 17.0-18.9 mm, the 19.0-21.0 mm group had higher adjusted odds of clinical pregnancy (aOR 5.58, 95% CI 1.31-23.84; p = 0.020) and live birth (aOR 15.15, 95% CI 1.74-131.95; p = 0.014), although both estimates were imprecise. Adjusted live-birth probabilities were 3.5%, 16.1%, and 9.1%, respectively. The direction of the live-birth association was preserved in the single-cycle sensitivity analysis (aOR 3.85, 95% CI 1.22-12.13; p = 0.021). Continuous linear and quadratic follicle-diameter models were not significant. Conclusions: The 19.0-21.0 mm category was associated with higher reproductive outcome rates, including live birth; however, the observational design, sparse events, wide confidence intervals, and null continuous analyses preclude defining an optimal trigger threshold. These findings are hypothesis-generating and require prospective multicenter confirmation.