Background: Experiences and conceptualisations of postnatal psychiatric illnesses can vary in mothers across cultures. This is important to understand in an era of superdiversity in the UK. Aims: To deepen the understanding of women's beliefs about the causes of postnatal psychiatric illnesses and women's lived experiences of a severe postnatal psychiatric illness. Methods: Two comprehensive qualitative systematic reviews using Noblit and Hare's meta-ethnography were conducted that explore a) women's beliefs about the causes of postnatal depression b) women's experiences of a severe postnatal psychiatric illness. The empirical research study uses Interpretative Phenomenological Analysis to explore 5 South Asian women's experiences of living with a severe postnatal psychiatric illness and experiences of Perinatal Mental Health Services. Findings: This thesis reports various explanatory models that can contribute to the onset of a postnatal psychiatric illness in new mothers. Not only was biology reported, but psychosocial, cultural, religious and spiritual factors. Mothers' experiences were also diverse and mothers from an ethnic minority background can have unique experiences that are shaped by their social and cultural milieu, which may impact help-seeking behaviour and treatment preferances. Discussion: To achieve culturally competent Perinatal Mental Health Services, it is imperative for clinicians to understand mothers' subjective understandings and experiences of their psychiatric illness.