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Background: Poor representation of pregnant and lactating women and people in clinical trials has marginalised their health concerns and denied the maternal–fetal/infant dyad benefits of innovation in therapeutic research and development. This mixed-methods systematic review synthesised factors affecting the participation of pregnant and lactating women in clinical trials, across all levels of the research ecosystem. Methods and findings: We searched 8 databases from inception to 14 February 2024 to identify qualitative, quantitative, and mixed-methods studies that described factors affecting participation of pregnant and lactating women in vaccine and therapeutic clinical trials in any setting. We used thematic synthesis to analyse the qualitative literature and assessed confidence in each qualitative review finding using the GRADE-CERQual approach. We compared quantitative data against the thematic synthesis findings to assess areas of convergence or divergence. We mapped review findings to the Theoretical Domains Framework (TDF) and Capability, Opportunity, and Motivation Model of Behaviour (COM-B) to inform future development of behaviour change strategies. We included 60 papers from 27 countries. We grouped 24 review findings under 5 overarching themes: (a) interplay between perceived risks and benefits of participation in women's decision-making; (b) engagement between women and the medical and research ecosystems; (c) gender norms and decision-making autonomy; (d) factors affecting clinical trial recruitment; and (e) upstream factors in the research ecosystem. Women's willingness to participate in trials was affected by: perceived risk of the health condition weighed against an intervention's risks and benefits, therapeutic optimism, intervention acceptability, expectations of receiving higher quality care in a trial, altruistic motivations, intimate relationship dynamics, and power and trust in medicine and research. Health workers supported women's participation in trials when they perceived clinical equipoise, had hope for novel therapeutic applications, and were convinced an intervention was safe. For research staff, developing reciprocal relationships with health workers, having access to resources for trial implementation, ensuring the trial was visible to potential participants and health workers, implementing a woman-centred approach when communicating with potential participants, and emotional orientations towards the trial were factors perceived to affect recruitment. For study investigators and ethics committees, the complexities and subjectivities in risk assessments and trial design, and limited funding of such trials contributed to their reluctance in leading and approving such trials. All included studies focused on factors affecting participation of cisgender pregnant women in clinical trials; future research should consider other pregnancy-capable populations, including transgender and nonbinary people. Conclusions: This systematic review highlights diverse factors across multiple levels and stakeholders affecting the participation of pregnant and lactating women in clinical trials. By linking identified factors to frameworks of behaviour change, we have developed theoretically informed strategies that can help optimise pregnant and lactating women's engagement, participation, and trust in such trials. Using a mixed-methods approach, Mridula Shankar and colleagues investigate the reasons why those who are pregnant and breastfeeding are under-represented in clinical trials. Author summary: Why was this study done?: Pregnant and lactating women and people are routinely excluded from participating in drug and vaccine clinical trials, resulting in limited options for prevention and treatment of medical conditions. Challenges to including pregnant and lactating women and people in clinical research have been identified at multiple levels of the research and health systems, but the full range of barriers and facilitators to participation are not well known. What did the researchers do and find?: We conducted a mixed-methods systematic review and identified 60 research articles from 27 countries on the views and experiences of pregnant and lactating women's participation in clinical research, from the perspectives of cisgender women, family and community members, health workers, and people involved in the conduct of clinical research. Using a thematic synthesis approach, we identified barriers affecting participation including women having a limited appetite for risk during pregnancy and lactation, concerns about women's bodily autonomy during pregnancy, and challenges in obtaining ethical approval for clinical research with pregnant women. We also identified facilitators of participation including the potential for personal health benefits, expectations of higher quality care, trust in the medical and research systems, and strong teamwork between researchers and health workers. What do these findings mean?: Our findings demonstrate the need for multipronged strategies to address barriers and reinforce facilitators across the various levels of the research and health systems. The actions that are needed to overcome these barriers and reinforce facilitators must be discussed, prioritised, and adapted to specific contexts. All included studies focused on factors affecting participation of cisgender pregnant women in clinical trials; future research should consider other pregnancy-capable populations, including transgender and nonbinary people. [ABSTRACT FROM AUTHOR]