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STUDY QUESTION Are intrauterine insemination (IUI) performance characteristics and post-processing total motile sperm count (TMC) related to live birth rate in couples with unexplained infertility? SUMMARY ANSWER Patient discomfort with IUI and lower inseminate TMC were associated with a reduced live birth rate, while time from hCG injection to IUI, sperm preparation method and ultrasound guidance for IUI were not associated with live birth success. WHAT IS ALREADY KNOWN We previously determined that some baseline characteristics of couples with unexplained infertility, including female age, duration of infertility, history of prior loss and income, were related to live birth rate across a course of ovarian stimulation and IUI treatment. However, the relationship between treatment outcomes and per-cycle characteristics, including ultrasound guidance for IUI, timing of IUI relative to hCG injection, difficult or painful IUI and inseminate TMC, are controversial, and most prior investigations have not evaluated live birth outcome. STUDY DESIGN, SIZE, DURATION This was a secondary analyses of 2462 cycles from the Assessment of Multiple Intrauterine Gestations from Ovarian Stimulation (AMIGOS) clinical trial. This prospective, randomised, multicentre clinical trial determined live birth rates following IUI after ovarian stimulation with clomiphene citrate, letrozole or gonadotropins in 854 couples with unexplained infertility. It was conducted between 2011 and 2014, and couples could undergo up to four consecutive treatment cycles. PARTICIPANTS/MATERIALS, SETTING, METHODS AMIGOS was an NIH-sponsored Reproductive Medicine Network trial conducted at 12 clinical sites. Participants were women with unexplained infertility who were between 18 and 40 years of age. Cluster-weighted generalised estimating equations (GEE), which account for informative clustering of multiple IUI treatment cycles within the same patient, were used to determine associations between IUI performance characteristics, including inseminate TMC, and live birth rate. Efficiency curves were also generated to examine the relationship between inseminate TMC and live birth rate. MAIN RESULTS AND THE ROLE OF CHANCE After adjustment for treatment group and baseline factors previously associated with live birth across a course of OS-IUI treatment, patient discomfort during the IUI procedure was associated with a reduction in live birth rate (aRR 0.40 (0.16-0.96)). Time from hCG trigger injection to IUI was not significantly associated with outcome. Higher TMC was associated with greater live birth rate (TMC 15.1-20.0 million (14.8%) compared to ≤5 million (5.5%)) (aRR 2.09 (1.31-3.33)). However, live births did occur with TMC ≤ 1 million (5.1%). LIMITATIONS, REASONS FOR CAUTION This investigation is a secondary analysis, and AMIGOS was not designed to address the present question. Since timed intercourse was allowed as part of the AMIGOS trial, we cannot rule out the possibility that any given pregnancy resulted from intercourse rather than IUI. WIDER IMPLICATIONS OF THE FINDINGS Most factors associated with the performance of IUI were not significantly related to obtaining live birth. Our findings suggest that higher TMC inseminated leads to an increase in live birth rate up to TMC ~20 million. However, there may be no reasonable threshold below which live birth is not possible with IUI. STUDY FUNDING/COMPETING INTEREST(S) Funding was received through grants from the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD): U10 HD077680, U10 HD39005, U10 HD38992, U10 HD27049, U10 HD38998, U10 HD055942, HD055944, U10 HD055936 and U10 HD055925. This research was made possible by funding by the American Recovery and Reinvestment Act. Dr Hansen reports grants from NIH/NICHD and Yale University during the conduct of the study, grants from Roche Diagnostics and grants from Ferring International Pharmascience Center US outside the submitted work. Dr Peck reports support from Ferring Pharmaceuticals outside the submitted work. Dr Coward has nothing to disclose. Dr Wild reports grants from NICHD during the conduct of the study. Dr Trussell has nothing to disclose. Dr Krawetz reports grants from NICHD during the conduct of the study, grants from Merck and support from Taylor and Frances and from Springer, outside the submitted work. Dr Diamond reports grants from NIH/NICHD, Yale University, during the conduct of the study and support from Advanced Reproductive Care AbbVie, Bayer and ObsEva, outside the submitted work. Dr Legro reports support from Bayer, Kindex, Odega, Millendo and AbbVie and grants and support from Ferring, outside the submitted work. Dr Coutifaris reports grants from NICHD/NIH and personal fees from American Society for Reproductive Medicine, outside the submitted work. Dr Alvero has nothing to disclose. Dr Robinson reports grants from NIH during the conduct of the study. Dr Casson has nothing to disclose. Dr Christman reports grants from NICHD during the conduct of the study. Dr Santoro reports grants from NIH during the conduct of the study. Dr Zhang reports grants from NIH during the conduct of the study and support from Shangdong University outside the submitted work. TRIAL REGISTRATION NUMBER n/a.