Dynamic Follicle Growth Rate Outperforms Traditional Ovarian Reserve Markers in Predicting Pregnancy in IUI Cycles

Background Accurate prediction of pregnancy outcomes in intrauterine insemination (IUI) cycles remains challenging. While follicular size and hormonal parameters are commonly used, the clinical value of follicular growth dynamics has not been fully elucidated. This study aimed to evaluate the association between follicular growth rate and pregnancy outcomes and to determine an optimal growth-rate threshold using multivariable modeling and ROC analysis. Methods This retrospective cohort study included 411 IUI cycles. Cycles were categorized according to follicular growth rate as < 1.5 mm/day or ≥ 1.5 mm/day. Clinical pregnancy was defined by positive serum β-hCG. Multivariable logistic regression was performed adjusting for PCOS status, anti-Müllerian hormone (AMH), endometrial thickness on the hCG trigger day, and timing of hCG administration. Receiver operating characteristic (ROC) curve analysis was used to assess discriminative performance and validate the optimal cut-off value. Results Clinical pregnancy rates were significantly higher in cycles with a follicular growth rate ≥ 1.5 mm/day compared with slower-growing follicles (19.8% vs 10.5%). In multivariable logistic regression analysis, follicular growth rate ≥ 1.5 mm/day remained independently associated with a higher likelihood of clinical pregnancy (adjusted OR 2.94, 95% CI 1.48–5.82; p = 0.002). AMH was also independently associated with pregnancy outcome, whereas PCOS status, endometrial thickness, and hCG trigger day were not. ROC analysis demonstrated moderate predictive performance (AUC = 0.61), with an optimal cut-off of 1.5 mm/day, yielding a sensitivity of 80.6% and a specificity of 41.6%. Conclusions Follicular growth rate is a strong and independent predictor of clinical pregnancy in IUI cycles. A growth velocity ≥ 1.5 mm/day represents a simple, non-invasive, and clinically applicable marker that may aid in optimizing hCG timing and improving cycle prognostication. Prospective studies are warranted to confirm whether growth-rate–guided interventions can improve live birth outcomes.