Background: Problems in intrapartum electronic fetal monitoring with cardiotocography (CTG) remain a major area of preventable harm. Poor understanding of the influences on safety may have hindered improvement. Taking an interdisciplinary perspective, we sought to characterise the everyday practice of CTG monitoring and the work systems within it takes place, with the goal of identifying potential sources of risk. Methods: Human factors/ergonomics (HF/E) experts and social scientists conducted 325 hours of observations and 23 interviews in three maternity units in the UK, focusing on how CTG tasks were undertaken, the influences on this work, and the cultural and organisational features of work settings. HF/E analysis was based on the SEIPS 2.0 model. Social science analysis was based on the constant comparative method. Results: CTG monitoring can be understood as a complex sociotechnical activity, with tasks, people, tools and technology, organisational and external factors all combining to affect safety. Fetal heart patterns need to be recorded and interpreted correctly. Systems are also required for seeking the opinions of others, determining whether the situation warrants concern, escalating concerns, and mobilising response. These processes may be inadequately designed or function sub-optimally, and may be further complicated by staffing issues, equipment and ergonomics issues and competing and frequently changing clinical guidelines. Practice may also be affected by variable standards and workflows, variations in clinical competence, teamwork and situation awareness, and the ability to communicate concerns freely. Conclusions: CTG monitoring is an inherently collective and sociotechnical practice.59 Improving it will require accounting for complex system interdependencies, rather than focusing solely on discrete factors such as individual technical proficiency in interpreting traces.