A US judge has declared a mistrial in the trial of Lindsay Clancy, a former nurse accused of murdering her three children.

Jurors in the case that has gripped the US and the world said they could not reach a unanimous verdict on whether she was criminally responsible when she strangled Callan, Dawson and Cora Clancy, who ranged from eight months to five years old.

The mistrial was reached after seven days of deliberations. The judge denied a request from the defence to hold a hearing over a juror who it said was the lone holdout on a verdict.

Prosecutors will be allowed to try Clancy again.

Lindsay Clancy’s lawyer did not dispute that she killed the children but said she should not be held criminally responsible because she was mentally ill.

Judge William Sullivan addresses the jury. Photo: AP

The livestreamed trial has brought attention to postpartum psychosis, a rare mental illness that Clancy’s attorney Kevin Reddington says she had.

The condition is linked to the stress, sleep deprivation and hormonal changes that follow childbirth. Reddington said she also has bipolar disorder.

After strangling her children, Clancy jumped from a second-story window and remains paralysed from the waist down.

Reddington sought to convince jurors that she should be found not guilty by reason of insanity, arguing that she ‌was a loving ‌mother who had been in the ⁠midst of a psychotic episode when she killed five-year-old Cora, three-year-old Dawson and eight-month-old Callan on January 24, ​2023.

Testimony from family members including Clancy’s now ex-husband, Patrick, detailed how she had struggled with her mental health in the preceding months following the birth of her third child, repeatedly seeking treatment from healthcare providers who prescribed her a myriad of drugs.

The trial drew comparisons to the case of Andrea Yates, a Texas woman who drowned her five children in a bathtub in 2001.

After an appeals court overturned her initial murder conviction, she was found not guilty by reason of insanity in 2006.

Her lawyers had said she suffered from ⁠severe postpartum psychosis.

Prosecutors did not dispute that Clancy, a former labour and delivery nurse at ‌Massachusetts General Hospital, ​had been dealing with mental health issues. But they argued that she was able to appreciate the wrongfulness of killing her children and intentionally chose to do so anyway.

They ​presented testimony from ‌a psychologist who, after examining Clancy, concluded that she had planned to commit suicide and chose to kill her children because “she was convinced that they would suffer without ​her”.

Prosecutors focused on evidence showing how shortly before the killings, she sent her husband out to pick up a food order and go to a pharmacy after mapping on her phone the time it would take for him to return home.

Patrick Clancy testified that his wife had been discharged from a psychiatric ​hospital ​less than two weeks earlier but showed no signs she planned to ​hurt herself or her kids in the days that followed.

She had a “normal” demeanour ‌when he left to pick up dinner, he said.

He spoke on the phone briefly with his wife while at the pharmacy.

Upon his return, he discovered her lying on the ground in the backyard with cuts.

She told him she had tried to take her own life.

He said he asked where the children were, and she said they were in the basement.

After he called 911 and first responders arrived, Patrick Clancy said he ran to the basement and found they were dead.

Several witnesses including Patrick Clancy testified that later, while Lindsay Clancy was at a hospital, she spoke about having heard a man’s voice tell her ​that if she did not act, she would ⁠lose her chance.

Neither Clancy, her family nor the jury showed any emotion as the judge announced the mistrial.

-with AAP