King's College London

Background: Midwifery continuity of care models are the only health system intervention associated with both a reduction in preterm birth (PTB) and an improvement in perinatal survival; however, questions remain about the mechanisms by which such positive outcomes are achieved. We aimed to uncover theories of change by which we can postulate how and why continuity of midwifery care models might affect PTB. Methods: We followed Pawson's guidance for conducting a realist review and performed a comprehensive search to identify existing literature exploring the impact of continuity models on PTB in all pregnant women. A realist methodology was used to uncover the context (C), mechanisms (M), and outcomes (O) and to develop a group of CMO configurations to illuminate middle-range theories. Results: Eleven papers were included from a wide variety of settings in the United Kingdom, Australia, and the United States. The majority of study participants had low socioeconomic status or social risk factors and received diverse models of midwifery continuity of care. Three themes—woman-midwife partnership, maternity pathways and processes, and system resources—encompassed ten CMO configurations. Building relationships, trust, confidence, and advocacy resulted in women feeling safer, less stressed, and more secure and respected, and encouraged them to access and engage in antenatal care with more opportunities for early prevention and diagnosis of complications, which facilitated effective management when compliance to guidelines was ensured. Organizational infrastructure, innovative partnerships, and robust community systems are crucial to overcome barriers, address women's complex needs, ensure quality of care, and reduce PTB risk. Conclusions: Pregnant women living in different contexts in the United Kingdom, Australia, and the United States at low and mixed risk of complications and with low socioeconomic status or social risk factors experienced continuity models in similar ways, and similar underlying mechanisms may have influenced PTB outcomes. Further research is required to understand how continuity models may influence behavioral change, physiological stress levels, ethnic disparities in PTB and care coordination, and navigation of health services.