The trial of Lindsay Clancy, a mother charged with murder in the strangulation deaths of her three young children, begins on Monday. After opening statements, her husband is expected to begin his testimony about Ms. Clancy’s deteriorating mental state and the subsequent killings of their children — Cora, 5, Dawson, 3, and Callan, 8 months.

The horrific deaths at their home in Duxbury, Mass., an affluent suburb of Boston, revived a debate about how the legal system should treat mentally ill mothers who kill their children.

At the trial, in state court in Plymouth, Mass., prosecutors are expected to say that Ms. Clancy, 35, carefully planned the killings, plotting to get her husband out of the house that night before choking their children with resistance bands.

Defense lawyers will say she was in the throes of a mental health crisis, overmedicated for severe postpartum mental illness and hearing voices that ordered her to kill herself and her children.

The Back Story

Ms. Clancy, who worked as a labor and delivery nurse, had experienced anxiety after the births of her first two children. Her symptoms returned after the birth of her third child in 2022 and intensified as she prepared to return to work. She sought treatment from a psychiatrist who prescribed her a growing list of medications.

In a lawsuit filed against the doctor this year, her husband, Patrick Clancy, argued that clinicians had failed to monitor her reactions to the medications “or attend to her worsening psychiatric condition.”

While taking more than a dozen medications — including Ambien, Klonopin, Valium, Prozac, Ativan, Zoloft and Seroquel — Ms. Clancy experienced worsening insomnia, anxiety and depression. By December 2022, she was reporting panic, numbness, racing thoughts, suicidal ideation and a growing fear “that something awful might happen,” according to the lawsuit.

Late that month, she admitted herself to an outpatient program for postpartum depression. On Jan. 1, 2023, she admitted herself to McLean Hospital, a residential psychiatric campus outside Boston, where she stayed for five days.

According to the lawsuit and medical records, Ms. Clancy’s crisis came to a head on Jan. 24, 2023, when she heard “a compelling and unrecognizable singular male voice” telling her that “she should die, that this is her last chance, and that her children would suffer if she was gone.”

That day, after ordering takeout food for dinner and asking her husband to pick it up, prosecutors say, she killed Cora and Dawson and fatally injured Callan, who died three days later.

Prosecutors say that Ms. Clancy cut her wrists and neck, jumped from a second-story window and fell about 20 feet to the ground, where her husband found her when he returned home less than 30 minutes later. She suffered traumatic spinal cord injuries and remains unable to walk, according to her lawyer.

The Husband’s Reaction

The searing events stunned the public. Then Mr. Clancy delivered another unexpected emotional jolt, releasing a wrenching written statement that supported his wife.

“My family was the best thing that ever happened to me,” Mr. Clancy wrote. “I took so much pride in being Lindsay’s husband and a dad to Cora, Dawson, and Callan.”

He described the quirks and talents of each child, and added: “They gave me purpose, and I never took it for granted. There is now a massive void where that purpose once was.”

Yet, Mr. Clancy wrote, his wife had been unfairly portrayed “by people who have never met her and never knew who the real Lindsay was.” Their marriage had been strong “and diametrically grew stronger as her condition rapidly worsened,” he said.

“I want to ask all of you that you find it deep within yourselves to forgive Lindsay, as I have,” Mr. Clancy added. “The real Lindsay was generously loving and caring towards everyone — me, our kids, family, friends, and her patients. The very fibers of her soul are loving.”

What Is Postpartum Psychosis?

About one in seven women are affected by postpartum depression, but research has been slow to investigate the causes.

Postpartum psychosis is a rarer, more severe illness carrying a high risk of suicide and infanticide.

Few women describe their experiences with either condition publicly, fearing condemnation in a culture in which any deviation from maternal bliss has been taboo.

The Trial

Ms. Clancy has pleaded not guilty to three counts of first-degree murder. She faces a penalty of life imprisonment without parole if convicted. Judge William Sullivan said at a hearing last week that he planned to seat 18 jurors, including six alternates, for a trial expected to last six to eight weeks.

Prosectors say she acted with “deliberate, premeditated and extreme atrocity and cruelty.”

Ms. Clancy consulted a mapping app before her husband left the house to determine how long he would be gone, they said in documents, and “gave herself the time and privacy needed to commit these murders.”

Kevin Reddington, the defense attorney representing Ms. Clancy, has said that her case ranked as “probably the most emotionally disturbing and challenging” of his decades-long career.

He predicted it would have a “significant impact on jurors.”