Inclusion Criteria:  Outpatient woman between ages 18 - 45 Within 6 months of delivery Have a DSM-5 diagnosis of MDD or BD I, BD II or other specified bipolar or related disorder with peripartum onset Have a score of >18 on the 17-item Hamilton Depression Rating Scale (HDRS) Have a score of ≤12 Young Mania Rating Scale (YMRS) at both the screening and baseline visits Able to communicate in English Capable of providing informed consent  Exclusion Criteria:  A diagnosis of schizophrenia spectrum or other psychotic disorders, obsessive-compulsive disorder, eating disorders, substance-related and addictive disorders At high risk for suicide (actively suicidal or a score of ≥ 3 on item #3 on the HDRS) Receiving a psychotropic drug such a mood stabilizer, an antidepressant or a sedative/hypnotic. Receiving psychotherapy Have a physical illness that is a contraindication to the use of quetiapine, or who have a history of intolerance or nonresponse to quetiapine Pregnant or planning on becoming pregnant during the study