RE104 Safety and Efficacy Study in Postpartum Depression

Inclusion Criteria: * Is ≤15 months postpartum at Screening. * Meet DSM-5 criteria for postpartum depression (PPD): experiencing a major depressive episode that began at any time starting at the beginning of the second trimester (≥14 weeks) of pregnancy through 4 weeks post delivery. * Has a Hamilton Depression Scale (HAM-D) total score meeting severity threshold at Screening and Baseline. * Is not using any psychotropic medications or psychotherapy for 30 days prior to Screening, OR are on an already stable/established regimen of SSRIs or psychotherapy for 30 days prior to Screening. * Has ceased breastfeeding at Screening. * Has a negative pregnancy test at Screening and Day 0 prior to study drug administration. Exclusion Criteria: * History or active postpartum psychosis per Investigator assessment. * History of treatment-resistant depression within the current postpartum depressive episode. * Has a significant risk of suicide. * Active or medical history of bipolar disorder, schizophrenia, schizoaffective disorder, psychotic disorder and/or borderline personality disorder, or first-degree family history of psychosis or bipolar disorder. * Medically significant condition rendering unsuitability for the study . * Has received electroconvulsive therapy (ECT) or transcranial magnetic stimulation within 90 days prior to Screening. * Has used psychedelics such as psilocybin, ayahuasca, mescaline, or LSD (with the exception of cannabis) within 12 months prior to Screening. * Has used or will need to use prohibited medications.