ABSTRACT Postpartum depression (PPD) is a debilitating and costly condition affecting over 20% of postpartum women and disproportionately impacts Blacks and Latinas. The COVID-19 pandemic is leading to a severe increase in the rate of depression while the US is already struggling with deep-seated inequities in mental health care for Black and Latina populations. The US Preventive Services Task Force recently recommended that women at increased risk of PPD should be referred for counseling to reduce the risk of PPD. However, numerous patient- level (e.g. lack of time or money) and system-level barriers (e.g. shortage of providers), now compounded with the COVID-19 pandemic, pose challenges to the receipt of counseling, particularly among Black and Latina populations that already experienced a lower rate of treatment initiation. There is an urgent need for evidence- based accessible and scalable mental health care options for these high-risk and vulnerable women. Mindfulness-Based Interventions (MBIs) are effective and theory-driven interventions for reducing depression symptoms. However, standard MBIs require 30+ hours of in-person or online sessions for 8 weeks and are often too intense for pregnant women with high depression symptom burden. Our pilot study of a self-paced, small dosage (10-20 minutes/day for 6 weeks), mobile-based (mHealth) MBI for pregnant women with high depression symptoms demonstrated promising feasibility and preliminary efficacy results in reducing depression symptoms providing support for a fully-powered trial. We will conduct a hybrid effectiveness- implementation study to test the effectiveness of the mHealth MBI to reduce postpartum depression symptoms in Black and Latina pregnant women at increased risk of PPD, and examine barriers and facilitators of implementation within a large healthcare system. The study will take place at Kaiser Permanente Northern California (KPNC), an integrated health care delivery system with a diverse population and comprehensive electronic health records. KPNC’s universal perinatal depression screening program will be leveraged to efficiently identify eligible women. We will randomize 600 pregnant (300 Latina and 300 Black) women into the intervention arm or time- and attention-matched control arm and ask them to practice mindfulness or another program for 6 weeks, 10-20 minutes a day. We will partner with HeadspaceTM and tailor the digital platform to deliver the mindfulness program and obtain adherence data. Patient-reported outcomes of depression (primary outcome), anxiety, stress and sleep (secondary outcomes), and attention monitoring and acceptance (target engagements) will be collected 3 times during the study (baseline, post intervention, and 4 weeks postpartum). Implementation of the intervention will be evaluated using the RE-AIM framework (Reach, Effectiveness, Adoption, Implementation and Maintenance). Further, using the results of this trial, we will determine optimal dosage a...