The jury began deliberations on Thursday in the trial of Lindsay Clancy, the Massachusetts woman charged with murder for strangling her three young children in 2023.

After more than four weeks of testimony that was by turns emotional, harrowing and combative, the lawyers on each side made their closing arguments to the jury as to whether or not Ms. Clancy should be held criminally responsible for the children’s deaths.

At the courthouse in Plymouth, Mass., on Thursday morning, Ms. Clancy dabbed her face with a tissue as her lawyer, Kevin Reddington, described her as “a darn good mother” and a respected nurse who had loved her children.

He read from a long list of medications that had been prescribed to Ms. Clancy in the months before the killings, arguing that she received “lousy medical care” when she sought help for her anxiety, and that her mental illness had spiraled out of control.

“This young lady is not guilty of the killing of her children,” Mr. Reddington said, “because she was suffering from a disease and defect.”

The judge, William Sullivan, told the jurors on Thursday morning that Ms. Clancy would not be criminally responsible if she had a mental disease or defect that prevented her from controlling her actions, or from distinguishing right from wrong.

In her closing argument, Jennifer Sprague, a prosecutor, presented the killings as a series of rational, premeditated choices, arguing that Ms. Clancy strangled the oldest children first because they were more likely to resist.

She called the killings an act of “extreme atrocity and cruelty” and described the horrific crime scene that Ms. Clancy left behind.

“She left them discarded on the basement floor, like broken toys, because she was done playing the game,” Ms. Sprague said. “You know, to a moral certainty, she is guilty.”

Ms. Clancy, 36, does not deny that she killed the children at her home in Duxbury, Mass., a Boston suburb. But Mr. Reddington has pursued an insanity defense, maintaining that she should not be held criminally responsible because she was suffering from postpartum psychosis and had experienced auditory hallucinations that day of a man’s voice ordering her to kill her children and then herself.

Prosecutors have argued that Ms. Clancy plotted to get her then-husband, Patrick Clancy, out of the house on Jan. 24, 2023. Mr. Clancy returned home from picking up takeout food to find the children in the basement with exercise bands around their necks, he said during his testimony. Cora, 5, and Dawson, 3, were dead. Callan, 8 months, died days later.

Mr. Clancy also found Ms. Clancy lying injured in the backyard. She had cut her wrists and neck and jumped from a second-story window of the home in what her lawyers have called a suicide attempt, which left her partially paralyzed.

Witnesses for the trial have also included Ms. Clancy’s mother, her sister and health professionals who treated Ms. Clancy before the killings or evaluated her afterward. The prosecution rested early last week after calling more than 70 witnesses, and the defense rested on Friday after calling 10.

Rebuttals, which give prosecutors an opportunity to respond to claims made by the defense, took up the last few days of the trial. The three witnesses for the prosecution, all forensic psychologists, pointed to evidence that Ms. Clancy may not have been suffering from postpartum psychosis.

The cross-examinations were combative. Mr. Reddington grilled the rebuttal witnesses and at one point asked the judge to grant a mistrial after a psychologist mentioned the concept of mortal sin while discussing Ms. Clancy’s Catholic upbringing.

On many days, demonstrators have gathered outside the courtroom. Many have been women, wearing pink to support Ms. Clancy, and to call attention to what they see as the health care system’s failure to care for new mothers with postpartum depression and other mental illnesses.

In her closing argument, Ms. Sprague said that the case was “not about our mental health system, or how it treats women.”

She added: “That’s a distraction to get you angry and passionate — about an important issue, but an issue that’s not on trial here.”

To secure a guilty verdict, the prosecutors must prove beyond a reasonable doubt not only that Ms. Clancy committed the crime, but also that she was criminally responsible for it — that she understood her actions.

Judge Sullivan also instructed the jury to choose among first-degree murder, second-degree murder and manslaughter if they find Ms. Clancy guilty. She faces life in prison without parole if convicted of first-degree murder.