Meghan Cliffel was eight months postpartum from the birth of her second daughter when she began hallucinating.

It was a Friday in 2015 and she had just finished her workday in New York City. She became convinced that people were following her, she thought she was being recruited for a cult, and she thought the world – including her husband – was out to get her and her two daughters.

Some 24 hours later, she was handcuffed to a gurney and taken to a psychiatric hospital.

She would go on to spend the next 12 days there receiving treatment for postpartum psychosis.

The condition often presents with mood symptoms such as irritability, mood swings, insomnia and what medical professionals would call a delirium-like state, Susan Hatters-Friedman, forensic psychiatry professor at Case Western Reserve University, told the BBC.

A patient often experiences psychiatric symptoms that appear rapidly, she said.

“Psychosis in general as a term means that the person’s out of touch with reality, and that often includes hallucinations, hearing things, seeing things that aren’t there, delusions, which are fixed false beliefs that are not consistent with your culture,” Hatters-Friedman said

On the day Clancy killed her children, she played with them outside building a snowman, her former husband, Patrick Clancy, said.

“She was having one of her best days,” he told the court during the trial.

Clancy had previously told her then-husband she was having thoughts of suicide or harming the children, but on that day everything seemed better, he testified.

When evening came that January 2023, he went out to grab dinner for the family of five. When he returned, he found his children dead and Clancy injured from a suicide attempt.

That contrast demonstrates the light-switch – or waxing and waning effect – that can be postpartum psychosis, experts told the BBC.

“A woman can appear fine one minute and be really ill a little bit later,” said Jennifer Payne, a reproductive psychiatry expert at the University of Virginia.

Experts say it’s not entirely clear what causes postpartum psychosis. It could be related to hormonal changes and stress after giving birth, and there seems to be a link between previous mental health challenges and developing the illness while postpartum.