A key question in a Massachusetts murder trial is whether postpartum psychosis drove a mother to kill her three children.On Monday, a jury heard opening statements in the trial of Lindsay Clancy, a former labor and delivery nurse at Massachusetts General Hospital. Both sides agree she strangled her children aged 5 and younger in 2023. But they disagree over whether she should be held criminally responsible.Clancy and her ex-husband, Patrick Clancy, filed lawsuits accusing her medical providers of failing to properly diagnose and treat her condition.Here’s what to know about the disorder.Postpartum psychosis is rare and dangerousIt’s estimated to afflict only 1 to 2 per 1,000 women after delivery. Experts at the Cleveland Clinic describe it as a mental health emergency affecting someone’s sense of reality. It can happen to anyone after giving birth, but the chances are higher for people with certain mental health conditions. In a journal article on the subject in 2023, researchers at the Medical University of Lublin in Poland said about half of people with postpartum psychosis have a previous history of psychiatric problems and the most common risk factor associated with the condition is bipolar affective disorder, which causes extreme mood swings.