Objective: Postpartum psychosis (PP) is a severe psychiatric disorder, with incomplete consensus on definition and diagnostic criteria. The Massachusetts General Hospital Postpartum Psychosis Project (MGHP3) was established to better ascertain the phenomenology of PP in a large cohort of diverse women spanning a wide geographical range (primarily in the US), including time of onset, symptom patterns, and associated comorbidities, psychiatric diagnoses pre- and post- the episode of PP, and also to identify genomic and clinical predictors of PP. This report describes the methods of MGHP3 and provides a status update., Method: Data are collected from women who experienced PP within 6 months of childbirth and who provided this information within ten years of the study interview. Subject data are gathered during a one-time structured clinical interview conducted by phone, which includes administration of the Mini International Neuropsychiatric Interview for Psychotic Disorders Studies (Version 7.0.2), the MGHP3© Questionnaire, and other information including lifetime mental health history and use of psychiatric medications both prior to the episode of PP and during the subsequent time period prior to study interview. Subjects also provide a saliva sample to be processed for genomic analyses; a neuroimaging assessment is also conducted for a subset of participants., Results: As of July 1, 2022, 311 subjects from 44 states and 7 countries were enrolled in MGHP3. Recruitment sources include social media, online advertisements, physician referral, community outreach, and partnership with PP advocacy groups., Conclusions: The rigorous phenotyping, genetic sampling, and neuroimaging studies in this sample of women with histories of PP will contribute to better understanding of this serious illness. Findings from MGHP3 can catalyze ongoing discussions in the field regarding proper nosologic classification of PP as well as relevant treatment implications., Competing Interests: I have read the journal’s policy and the authors of this manuscript have the following competing interests: Lee S. Cohen, MD – as of 1/20/2023 Research Support: Dr. Cohen is an employee of Massachusetts General Hospital, and works with the MGH National Pregnancy Registry. MGH National Pregnancy Registry: Current Sponsors: Alkermes, Inc. (2016-Present); Eisai, Inc. (2022-Present); Johnson & Johnson/Janssen Pharmaceuticals, Inc (2019-Present); Otsuka America Pharmaceutical, Inc. (2008-Present); Sage Therapeutics (2019-Present); Supernus Pharmaceuticals (2021-Present); Teva Pharmaceutical Industries Ltd. (2018-Present). As an employee of MGH, Dr. Cohen works with the MGH CTNI, which has had research funding from multiple pharmaceutical companies and NIMH. Other research support: Brain & Behavior Research Foundation; National Institute on Aging; National Institutes of Health; SAGE Therapeutics Advisory/Consulting: JDS Therapeutics LLC Speaking/Honoraria: None Royalty/patent, other income: None Marlene P. Freeman, MD – as of 1/20/2023 Research Support: Dr. Freeman is an employee of Massachusetts General Hospital, and works with the MGH National Pregnancy Registry. MGH National Pregnancy Registry: Current Sponsors: Alkermes, Inc. (2016-Present); Eisai, Inc. (2022-Present); Johnson & Johnson/Janssen Pharmaceuticals, Inc (2019-Present); Otsuka America Pharmaceutical, Inc. (2008-Present); Sage Therapeutics (2019-Present); Supernus Pharmaceuticals (2021-Present); Teva Pharmaceutical Industries Ltd. (2018-Present). As an employee of MGH, Dr. Freeman works with the MGH CTNI, which has had research funding from multiple pharmaceutical companies and NIMH. Other Research Support: SAGE Therapeutics, JayMac. Advisory/Consulting, Data Safety Committees: Independent Data Safety and Monitoring Committees: Janssen (Johnson& Johnson), Novartis, Neurocrine; Advisory Boards: Eliem, Sage; Brainify; Everly Health; Tibi Health; Relmada; Beckley Psytech Speaking/Honoraria: WebMD, Medscape, Pri-Med, Postpartum Support International Royalty/patent, other income: Scale Royalties- through MGH Scale, The Massachusetts General Hospital Female Reproductive Lifecycle and Hormones Questionnaire (Freeman et al. 2013) This does not alter our adherence to PLOS ONE policies on sharing data and materials., (Copyright: © 2023 Cohen et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)