Optimal follicle size for human chorionic gonadotropin trigger in mono-follicular growth during letrozole-induced intrauterine insemination cycles: findings from a large retrospective study.

OBJECTIVE

To determine the optimal lead follicle size for human chorionic gonadotropin trigger in letrozole intrauterine insemination (IUI) cycles, focusing on pregnancy outcomes.

DESIGN

Retrospective cohort study.

SUBJECTS: 724 letrozole-IUI cycles conducted at Omega Fertility Clinic between January 1, 2018, and September 30, 2023.

EXPOSURE: Patients received letrozole for ovulation induction from cycle days 3-7, with follicle development monitored via transvaginal ultrasound. Human chorionic gonadotropin was administered when a lead follicle reached the desired size, followed by IUI.

MAIN OUTCOME MEASURES: The primary outcome was the clinical pregnancy rate. Lead follicle sizes were categorized (≤18 mm, 19-23 mm, ≥24 mm) and dichotomized using thresholds from the receiver operating characteristic curve and Youden's Index.

RESULTS

Among 724 cycles, 92 resulted in clinical pregnancy. Clinical pregnancy rates were 8.45% for follicles ≤17 mm, 8.89% for 18 mm, and increased from 12.92% (19 mm) to 18.52% (22 mm) before declining to 11.43% for follicles ≥24 mm. Biochemical pregnancy rates followed a similar trend. Logistic regression revealed significantly higher odds of clinical pregnancy for follicles 19-23 mm (adjusted odds ratio [OR], 1.71; 95% confidence interval [CI], 1.01-3.03) compared with ≤18 mm. Follicles ≥24 mm had an adjusted OR of 1.80 (95% CI, 0.98-3.31), narrowly crossing the null, suggesting a possible effect despite not reaching statistical significance. The optimal threshold for follicle size was 19 mm, based on receiver operating characteristic curve analysis. Dichotomized analysis confirmed higher odds of clinical pregnancy for follicles ≥19 mm (adjusted OR, 1.74; 95% CI, 1.01-3.01).

CONCLUSION

The optimal lead follicle size for human chorionic gonadotropin trigger in letrozole-IUI cycles is 19-23 mm, significantly improving clinical pregnancy rates. Follicles ≥24 mm may also yield positive outcomes and warrant further investigation. These findings provide evidence-based guidance for optimizing letrozole-IUI treatments.