PROJECT SUMMARY Depression is one of the most common perinatal complications, with 1 in 7 mothers qualifying for a diagnosis of postpartum depression (PPD). Sexual and gender minority (SGM) people experience PPD at comparable or higher rates as cisgender, heterosexual people. Yet, extremely little is known about risk factors for PPD that are specific to SGM people (e.g., minority stress) and there are no studies of PPD prevention or treatment among SGM pregnant people. This project addresses these major gaps in knowledge that limit available, effective services for preventing PPD among SGM pregnant people. It tests associations between minority stress, general stress (i.e., cortisol), and PPD symptoms. It also examines, quantitatively and qualitatively, the benefit of a universal (i.e., not SGM tailored) perinatal preventive intervention for PPD, delivered in English and Spanish, for SGM pregnant people. The PPD prevention program, ROSE, is designed to increase access, scalability, and address the mental health needs of underserved populations (e.g., racial and ethnic minority people). This administrative supplement will leverage data collected in the parent R01, a randomized clinical trial that is testing a virtual version against the in-person version of an evidence-based preventive intervention in a randomized controlled trial (RCT). Data collected as part of this administrative supplement guided by minority stress and LGBTQ health equity theories in every aim, will produce: (1) basic science on PPD among SGM and (2) a mixed- methods evaluation of an efficacious PPD prevention program among SGM pregnant individuals. This project will test the following aims: 1: Incorporate measures of stress relevant for sexual and gender minority individuals across levels of analysis and examine associations of SGM-specific stress (i.e., minority stress) and general stress (i.e., cortisol) with PPD symptoms. 2: Test if the hospital-based PPD prevention program is as effective for SGM participants as for their cisgender, heterosexual counterparts, if the same hypothesized mediators improve for SGM participants (e.g., social support), and examine if program effects are moderated by SGM minority stress 3: Explore through rich qualitative data, program experiences, benefits (e.g., social support), and unmet needs among SGM participants who completed the ROSE program, including across intersections of minoritized SGM and racial/ethnic identities. This team, led by MPIs Davis and Rhoades, with support from Co- I Perry, an expert in SGM health, will ensure this project’s success. The discoveries generated by this supplement will narrow gaps in our understanding of how to effectively prevent PPD for SGM pregnant people.