A 36-year-old woman comes to the physician with low mood, anxiety, trouble sleeping at night, constant fatigue, and strong, inexplicable feelings of guilt and hopelessness for the past 20 days. She says that she does not feel like doing anything most days, even things that used to previously make her happy like being with friends or listening to music. Instead, she cries often, or feels “on edge.” She spends most of the day in bed. She also worries that she might harm her 3-week-old baby because she frequently feels irritable and apathetic towards him. Her partner says that the patient experienced similar but less severe symptoms soon after the birth of their first child, which resolved spontaneously. Although the patient herself has never been diagnosed with a psychiatric condition, her mother suffers from bipolar mood disorder. General and systemic examinations reveal no abnormalities. What is the most likely diagnosis?