BACKGROUND: Suicide is a leading cause of maternal death during pregnancy and up to a year after birth. Psychological and psychosocial risk factors for maternal suicide ideation and behaviour have been identified but do not account for why mothers begin to experience suicidal thoughts. Qualitative research offers a way of identifying what might drive mothers to initially consider suicide and then go on to act on such thoughts; crucial for the development of assessments to identify, and interventions to target, maternal suicide ideation and behaviour. We aimed to develop a grounded theory outlining what makes women think about suicide and/or engage in suicidal behaviour during pregnancy and the first 12 months following birth? METHOD: Semi-structured interviews were conducted with 12 mothers in the UK who had suicidal thoughts during pregnancy and/or the first year following birth. A constructivist approach to grounded theory was adopted which guided the data collection and analysis processes. RESULTS: We developed a model outlining the theorised process of psychological factors that culminates in mothers experiencing suicidal thoughts and then making a suicide attempt during the perinatal period. The process was initiated when mothers felt attacked by motherhood which led to feeling like a failure, self-identifying as a "bad mother" and subsequent appraisals of entrapment and/or defeat. When nothing resolved the distress and as mothers collated reasons for why they perceived they needed to die, suicidal behaviour became a viable and appealing option. We theorised that mothers might make a suicide attempt when they entered a state of intense "darkness" brought on by a trigger, followed by a temporary lapse in the conflict between the desire to live and desire to die and an opportunity to attempt. CONCLUSIONS: Participants stressed the rapid onset of suicidal thoughts. We suggest that healthcare professionals enquire about the mother's feelings towards the baby and of isolation, how she views herself as a mother, feelings of entrapment and defeat during routine contacts to aid identification and prevention of suicidal ideation/behaviour. Suggested interventions to prevent suicidal thoughts and behaviour include helping women manage their expectations for pregnancy and the postpartum period.