Objective: To consider how psychosocial assessment in the perinatal period may act as an intervention. Background: Psychosocial assessment has been introduced into routine antenatal care in several countries but there has been no consideration of 'measurement reactivity', the effects of such processes on those being measured. Methods: Psychosocial assessment as part of routine antenatal booking and by self-completion of a research questionnaire, followed by interview of a purposive sample of 22 women who scored above threshold on maternal stress measures. Interviews were conducted up to three times during pregnancy and the early postnatal period, to explore women's experiences and understandings of maternal stress and to obtain their views on antenatal psychosocial assessment and social support. Transcribed data and field notes were analysed using Framework Analysis. Results: The potential for assessment to act as an intervention varied across different settings (clinical practice and assessment in a research context), with different methods (self-completion and assessment as part of a consultation or interview), and across individuals. Measurement effects were pronounced through longitudinal involvement, interviewer style and concurrently assessing the origins of psychological health and coping strategies. Conclusion: The analysis illustrates how reflexivity can enable new and often unexpected findings to emerge. The findings raise important questions about how the role of the research(er) is constructed, particularly when evaluating interventions. Clinically, the findings have implications for how we view psychosocial assessment within care pathways.