ObjectiveThe objective of this study was to estimate whether the time interval between two intrauterine inseminations (IUI) treatments needs to be extended by one menstrual cycle or more in patients undergoing successive cycles of ovulation stimulation, and whether this will have an impact on the clinical pregnancy rate (CPR).Study DesignRetrospective cohort study.Study siteThe study site was the reproductive medicine center of a teaching hospital.Patient(s)The subjects were women and their husbands who received two or more intrauterine insemination in our reproductive medicine center due to mild infertility in the period from January 2017 to December 2019. Patients were divided into 2 groups according to the number of days between the last menstrual day(LMD)and the previous IUI operation day(POD), continuous group (the time from the LMD to POD ≤ 34 days) and delayed group (the time from the LMD to POD ≥ 35 days). We excluded cycles with intervals of more than 180 days.In order to avoid the inclusion of multiple repeat cycles for the same couple, only the first two cycles of IUI treatment in the same couple were allowed to be included in this study. That is, when they failed the first IUI cycle, they were given a second IUI treatment.Intervention(s)No intervention.Main Outcome Measure(s)A total of 550 cycles met the inclusion criteria, and 374 (68.0%) cycles met the inclusion criteria for the continuous group,the remaining 176 (32.0%) cycles with at least one or more menstruations between two IUI cycles were included in the delayed group.The primary outcome measure was clinical pregnancy rate (CPR), with secondary outcomes including abortion rate. Differences in clinical pregnancy rate (CPR)、abortion rate were compared between the two groups.ResultThere was no significant difference between the continuous group and the delayed group in female age, male age, infertility duration, infertility type, female BMI, endometrial classification, endometrial thickness, semen volume before treatment, sperm density before treatment, percentage of forward motile sperm before treatment, sperm density after treatment, and percentage of forward motile sperm after treatment. There were no statistical differences between the delayed group vs continuous group regarding the clinical pregnancy rate (20.5 % vs 21.9 %) and abortion rate (27.8% vs 22.0%)（P>0.05）. The above factors were included for binary logistic regression analysis. It was found that the increase of endometrial thickness promoted the clinical pregnancy rate, which was statistically significant (OR=1.205, 95% CI 1.05-1.384,P=0.008). Compared with primary infertility, secondary infertility can promote the improvement of clinical pregnancy rate, which is statistically significant (OR=2.637,95%CI 1.313-5.298,P=0.006). The effect of time interval between IUI on clinical pregnancy was not statistically significant (OR=1.007,95% CI 0.513-1.974,P=0.985).ConclusionsOverall, prolonging the interval between two IUI did not significantly improve pregnancy outcomes. Unless there are clear clinical indications, it is not necessary to deliberately prolong the interval between two treatments.