Background: Although the number of follicles at intrauterine insemination (IUI) is associated with the pregnancy rates and multiple pregnancy rates. Multiple pregnancy rates are low in older women. Therefore, this study was undertaken to determine the clinical pregnancy rate of IUI in women 38–43 years of age based on the number of stimulated mature follicles. Methods: A retrospective cohort study was performed including all the first to third stimulated IUI cycles conducted after the age of 38 years in a single academic fertility center between January 2011 and March 2018. Results: A total of 1574 IUI cycles were included in the study. The patients were divided according to the number of mature follicles (>14 mm in diameter) at the last ultrasound before the human chorionic gonadotropin (hCG) trigger. The total pregnancy rate was 9.1% and only 5 multiple pregnancies occurred. The parity (p = 0.049), the number of follicles 10–14 mm (p = 0.002), and the peak endometrial thickness (p = 0.003) were significantly different between the groups. No statistical difference was observed between the groups regarding pregnancy rates (p = 0.93) and clinical pregnancy rates (p = 0.21). Multivariate logistic regression controlling for confounding effects comparing clinical pregnancy rates with the standard as 1 follicle 14 mm or greater as benchmark did not alter the results. Conclusions: In women 38 to 43 years of age undergoing controlled ovarian hyperstimulation (COH)/IUI, one mature follicle yielded similar pregnancy and clinical pregnancy rates compared to multiple follicles, possibly due to the aneuploidy rate at this age.