Background: The most widely used copper intrauterine device (IUD) in the world (the TCu380A), and the only product available in many countries, causes side effects and early removals for many users. These problems are exacerbated in nulliparous women, who have smaller uterine cavities compared to parous women. We compared first-year continuation rates and reasons/probabilities for early removal of the TCu380A versus a smaller Belgian copper IUD among nulliparous users., Methods: This 12-month interim report is derived from a pre-planned interim analysis of a sub population and focused on key secondary comparative endpoints. In this participant-blinded trial at 16 centres in the USA, we randomised participants aged 17-40 in a 4:1 ratio to the NT380-Mini or the TCu380A. In the first year, participants had follow-up visits at 6-weeks and 3, 6, and 12-months, and a phone contact at 9 months; we documented continued use, expulsions, and reasons for removal. Among participants with successful IUD placement, we compared probabilities of IUD continuation and specific reasons for discontinuation using log-rank tests. This trial is registered with ClinicalTrials.gov number NCT03124160 and is closed to recruitment., Findings: Between June 1, 2017, and February 25, 2019, we assigned 927 nulliparous women to either the NT380-Mini ( n = 744) or the TCu380A ( n = 183); the analysis population was 732 (NT380-Mini) and 176 (TCu380A). Participants using the NT380-Mini, compared to the TCu380A, had higher 12-month continuation rates (78·7% [95% CI: 72·9-84·5%] vs. 70·2% [95% CI: 59·7-80·7], p = 0·014), lower rates of removal for bleeding and/or pain (8·1% vs. 16·2%, p = 0·003) and lower IUD expulsion rates (4·8% vs. 8·9%, p = 0·023), respectively., Interpretation: The NT380-Mini offers important benefits for a nulliparous population compared to the TCu380A in the first twelve months, when pivotal experiences typically occur. Higher continuation rates with the NT380-Mini may avert disruptions in contraceptive use and help users avoid unintended pregnancy., Funding: Bill & Melinda Gates Foundation, Eunice Kennedy Shriver National Institute of Child Health and Human Development, and Mona Lisa, N.V. (Belgium)., Competing Interests: DH, PLC, and KN's institution (FHI 360) received funding from Bill & Melinda Gates Foundation and Mona Lisa, N.V. to conduct this study. The following investigators’ institutions received contract funding from FHI 360 (with funds originating from Bill & Melinda Gates Foundation) to help conduct this study: CAS, ID, BAC, ABG, CD, ALN. The following investigators’ institutions received contract funding from Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) to help conduct this study: DKT, JTJ, MDC, KOW, SBT, JLK, CD, MAT, DFA, PMC, AEB. These two employees of the NIH were supported by Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) to help conduct this study: DLB and JEB. No author received personal financial payments for this work. No author has financial relationships with Mona Lisa, N.V. DH reports board membership on Society of Family Planning. CAS reports contracts or grants to institution from Bayer Pharma, Sebela Pharma, and Danco Pharma; royalties to self and institution from Atheneum Pharma; payment for expert testimony from Center for Reproductive Rights, Planned Parenthood Federation and ACLU; a U.S. Provisional Patent Application No. 62/777,369; honoraria from American Board of Obstetrics & Gynecology. DKT reports contracts or grants to institution from The Laura and John Arnold Foundation, Society of Family Planning Research Fund, the William and Flora Hewlett Foundation, the Willard L. Eccles Foundation, the Intermountain Community Care Foundation, Bayer Women's Health, Organon, Cooper Surgical, Medicines360, Sebela Pharmaceuticals and anonymous foundation. MCD reports contracts to institution from Chemo Research SL, Evofem, HRA Pharma, Medicines360, Merck, Sebela; consulting fees from Estetra SRL, FHI360, Libbs, Mayne, Medicines360; speaking honorarium from Gedeon Richter, Mayne; payment/reimbursement of expenses for travel to meetings to present research from Medicines360; payment/reimbursement of expenses for travel to meetings to present research or attend Advisory Board from Estetra SRL; payment/reimbursement of expenses for travel to attend Advisory Board from TherapeuticsMD; Advisory Board meetings with honorarium from Evofem, Fuji Pharma, Mayne, Merck, Searchlight and TherapeuticsMD; honorarium for duties as Deputy Editor of the journal Contraception. KOW reports grants or contracts to institution from Bayer Pharmaceuticals and honoraria for a remote consultancy from Bayer Pharmaceuticals. SBT reports contracts to institution from Sebela, Medicines 360, Bayer Healthcare, Chemo Research, S.L., Merck & Co.; DSMB for Merck & Co.; Advisory Board for Bayer Healthcare; Society of Family Planning board member; American Board of Obstetrics & Gynecology (Complex Family Planning Division Member). BAC reports contracts or grants to institution from Medicines360, Sebela, and Mylan; member of American College of Obstetrics and Gynecology working group. ABG reports grants or contracts to institution from Merck; royalties from Uptodate; consulting fees from Society of Family Planning; board member of Society of Family Planning. ALN reports contracts or grants to institution from Merck, Mylan, Myovant, Sagami Rubber Industries, Sebela; consulting fees from Agile Therapeutics, Bayer HealthCare, Mayne Pharma, Pfizer, TherapeuticsMD; payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events from Agile Pharma, Bayer HealthCare, Myovant Sciences, Merck, TherapeuticsMD; support for attending meetings and/or travel from Agile Pharma, Bayer HealthCare, Myovant Sciences, Merck, Pfizer, TherapeuticsMD. MAT reports Board member of American Society for Reproductive Medicine. DFA reports grants or contracts to institution from Bayer Healthcare, Dare Biosciences, Estetra, Myovant, and ObsEva; consulting fees from Bayer Healthcare, Exeltis, Mithra, Lupin, ObsEva, Mithra; honoraria for lecture from Exeltis; DSMB member; Board of directors for Diczfalusy Foundation; stock ownership in InnovaGyn, Inc.; stock options with Agile Therapeutics. PMC reports consulting fees from Bayer to provide remote IUD consultative support to clinicians; payments to institution from Bayer for clinical research. AEB reports grants or contracts to institution from Merck & Co, Inc (USA) and Scope/Chemo (Spain); honorarium and travel to meetings from TherapeuticsMD; support for related work and travel as associate editor for ACOG Clinical Updates; support for travel/honorarium from American Board of Obstetrics & Gynecology. BK reports grants or contracts to institution from Contramed Pharmaceuticals (Sebela), Evofem Biosciences, Gynuity Health Projects; royalties from Uptodate; consulting fees from Merck. The remaining authors do not have any related relationships to disclose: JTJ, KN, ID, PLC, JLK, CD, JEB, DLB., (© 2022 The Author(s).)