Materials and Methods: This retrospective descriptive-analytical study included 211 infertile couples undergoing 335 IUI cycles at Kamali Hospital, Karaj, between 2018 and 2021. Ovarian stimulation was mainly performed with clomiphene citrate, with gonadotropin supplementation in selected cases. Inclusion criteria required at least one patent fallopian tube and acceptable semen parameters (motility >20%, count >4 million, morphology ≥4%). Effect sizes (mean differences with 95% confidence intervals) were calculated in addition to p-values.

Results: The overall success rate was 20.4% per couple and 12.8% per cycle. Endometrial thickness was significantly higher in successful cycles (8.3 ± 0.9 mm) than in unsuccessful ones (7.4 ± 1.7 mm), mean difference 0.9 mm (95% CI: 0.4–1.4, P = 0.002). Shorter infertility duration was also associated with success (3.2 ± 2.1 vs. 4.0 ± 2.5 years, mean difference −0.8 years, 95% CI: −1.4 to −0.2, P = 0.007). No significant associations were found with maternal age, BMI, sperm count, motility, or morphology.

Conclusions: Endometrial thickness and infertility duration were the strongest predictors of IUI success. Optimizing uterine receptivity and considering infertility duration in patient selection may improve outcomes.