Objective: Partners of birthing mothers can themselves experience perinatal mental health (PMH) difficulties. Despite birth rates increasing amongst LGBTQIA+ communities and the significant impact of PMH difficulties, this area is under-researched. This study aimed to examine the experiences of perinatal depression and anxiety of non-birthing mothers in female same-sex parented families. Design: Interpretative Phenomenological Analysis (IPA) was used to explore the experiences of non-birthing mothers who self-identified as having experienced perinatal anxiety and/or depression. Setting and participants: Sevenparticipants were recruited from online and local voluntary and support networks for LGBTQIA+ communities and for PMH. Interviews were in-person, online or via telephone. Measurements and findings: Six themes were generated. Distress was characterised by feelings of “Failure and Inadequacy in Role” (i.e., parent, partner and individual) and “Powerlessness and Intolerable Uncertainty” in their parenting journey. These feelings were reciprocally influenced by perceptions of the “Legitimacy of (Di)stress as a Non-birthing Parent”, which impacted help-seeking. Stressors that contributed to these experiences were: “Parenting Without” a parental role template, social recognition and safety, and parental connectedness; and “Changed Relationship Dynamics” with their partner. Finally,participants spoke about “Moving Forward” in their lives. Key conclusions: Some findings are consistent with the literature on paternal mental health, including parents’ emphasis on protecting their family and experiencing services as focusing on the birthing parent. Others appeared distinct or amplified for LGBTQIA+ parents, including the lack of a defined and socially recognised role; stigma concerning both mental health and homophobia; exclusion from heteronormative healthcare systems; and the importance placed on biological connectedness. Implications for practice: Culturally competent care is needed to tackle minority stress and recognise diverse family forms.