Abstract: Background: Being born before full gestation can have short-term and life-long health implications, yet it remains difficult to determine the risk of preterm birth among expectant mothers. Across different health settings, increasing attention is given to the health and behavioural consequences of adverse childhood experiences (ACEs) such as child abuse or neglect, or exposure to harmful household environments (e.g. in which caregivers abuse alcohol), and the potential value of understanding these hidden harms when supporting individuals and families. A large international evidence base describes the association between childhood adversity and early years outcomes for mothers and children. However, the relationship between maternal ACEs and preterm birth has received far less attention. Methods: Secondary analysis was carried out on anonymised cross-sectional data from health visiting services in south and west Wales that had previously captured information on mothers’ ACEs during routine contacts. Demographic data and information on mothers’ health were extracted from the Healthy Child Wales Programme. Results: Half of all mothers sampled had experienced at least one ACE, with a history of ACEs more common among younger, white British mothers and those residing in deprived areas. Preterm birth was significantly independently associated with retrospective reports of childhood sexual abuse (adjusted odds ratio [AOR] = 3.83, 95% confidence interval [CI] = 1.19–12.32, p = 0.025), neglect (AOR = 7.60, 95%CI = 1.81–31.97, p = 0.006) and overall ACE exposure (AOR = 2.67, 95%CI = 1.14–6.23, p = 0.024), with one in ten mothers (10.0%) who experienced ≥4 ACEs having preterm birth. Sub-analyses revealed a more pronounced relationship among mothers with no known chronic health conditions, with those with ≥4 ACEs and no known chronic condition four times more likely to give birth preterm (AOR = 3.89, 95%CI = 1.40–10.80, p = 0.009). Conclusions: Findings highlight the importance of the entire maternal experience. The experience of childhood adversity can have a lasting impact into and beyond the prenatal period, potentially increasing the risk of preterm birth, even among otherwise healthy women. Increasing our understanding of the potential perinatal outcomes associated with ACEs can help to inform how maternity services and partners offer trauma-sensitive support to mitigate some of the risks of early parturition, as well as target intergenerational cycles of adversity and poor health.