Introduction: Variation in outcomes reported in maternity-related clinical trials and practice stifles data synthesis and contributes to ineffective or harmful treatments and interventions. Variation can be addressed using core outcome sets (COSs), minimum agreed sets of outcomes that should be measured and reported in all clinical trials in a specific area of health or health care. This scoping review identified studies that developed maternity-related COSs; evaluated the extent, scope, quality, and consistency of outcomes across similar COSs; and identified current gaps in evidence. Methods: A multifaceted search of 2 COS registers (Core Outcome Measures in Effectiveness Trials, Core Outcomes in Women's and Newborn Health), the International Consortium for Health Outcomes Measurement website, electronic databases (MEDLINE, Embase, CINAHL), and hand search was conducted. Published, English-language studies describing maternal and neonatal COSs for any health condition and published from inception to January 2020 were included. COS development process was evaluated against recently published COS Standards for Development: scope, stakeholder involvement, and consensus process. Results: Thirty-two articles relating to 26 COSs were included (maternal: 18 articles that addressed 17 COSs; neonatal: 14 articles that addressed 9 COSs) and covered a range of obstetric and neonatal conditions. COSs were published between 2006 and 2020, 58% since 2017. Maternal COSs included more outcomes (median, 17; range, 50) than neonatal COSs (median, 8; range, 20). Overlap in COSs was seen for maternity care and gestational diabetes. Overlap in outcomes was seen across similar COSs, which were mostly inconsistent or poorly defined. No included COS met all minimum standards for development. Two COSs extended recommendations for how and when to measure outcomes. Discussion: Growth in COS development in the last 3 years signifies increasing commitment to address variation and improve data synthesis. Although the quality of the development process has improved in the last 3 years, there is a need for improvement. This article presents an urgent need to minimize overlap in outcomes and standardize outcome measurement, case definitions, and timing of measurement between COSs.