Background: Although widely acknowledged that adequate maternal nutrition is important for mother and baby, limited research has focussed on women with severe mental illnesses (SMI) in pregnancy. The present study reports on nutritional factors and food choices and investigates barriers and facilitators to healthy nutritional choices by pregnant women with SMI. Methods: A prospective mixed method study was undertaken of 38 pregnant women with SMI, including a cross-sectional survey, a food frequency questionnaire and 12 postnatal qualitative interviews, with integrated analysis of all data. Results: Elevated rates of obesity (35%) were found, with 82% of women having above the recommended gestational weight gain. Despite perceived knowledge, 32% of women did not meet any of the Five Food Group serving recommendations for pregnancy and consumed above-recommended levels for processed (19%) and sugar snacks (51%). Thematic analysis identified four main barriers: a discrepancy between knowledge and action, food cravings, mental health, and physical health. During pregnancy, food cravings were reported in 66% of women, psychological distress in 71% and physical distress in 37%. Screening identified 19% with potential eating disordered behaviours. Despite the challenges, several facilitators were identified and covered three themes: access to a dietitian, information delivery and support, and comprehensive care. Discussion: Women with SMI in pregnancy struggle with issues of obesity, gestational weight gain, food cravings and possible eating disorder behaviours. They have additional challenges when pregnant, with management of their mental health and physical health having a direct impact. Interventional strategies in this population should incorporate findings from this research.