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.