Prevalence of maternal obesity is increasing, with health risks for mother and infant. Effective health promotion depends on sufficient knowledge and appropriate communication skills. We aimed to explore women's, midwives' and health visitors' perceptions of current practice in helping women manage their weight and supporting healthy behaviour change during pregnancy, and their perceived training needs. A modified grounded theory methodology was adopted, based upon critical realist assumptions. Following consultation events with fifty six practitioners to inform data collection tools, twenty (different) practitioners and nine women participated in focus groups. Comparative analysis generated four themes: A core theme, "Discouraging discourses", described health professionals' negative beliefs and reactive approach to communicating about weight. "Staff resources" identified limitations in and requirements for practitioner knowledge, skills and tools for effective communication. "Contextual influences" were social factors, which hindered practitioners' efforts to achieve healthy behaviour change. "Communicating as a Team" identified the importance of and challenges to a team approach. Findings have implications for weight management in pregnancy, practitioner resources, teamwork, and national health promotion campaigns.