A federal judge has granted the ‘Womb Raider’ killer a stay of execution, pending a competency hearing.
A date has not yet been set for the competency hearing.
Montgomery, 52, murdered a pregnant woman before cutting her unborn baby out of her womb and kidnapping her.
She received the death sentence for killing 23-year-old Bobbie Jo Stinnett in Missouri, US in December 2004.
Montgomery, the only woman on federal death row, would have been the first woman executed since 1953.
She was due to receive a lethal injection at Terre Haute prison in Indiana – the only federal jail with an active death chamber.
Court for the Southern District of Indiana granted her stay.
According to CNN, he said: “Ms. Montgomery’s motion to stay execution is GRANTED to allow the Court to conduct a hearing to determine Ms. Montgomery’s competence to be executed.”
Prosecutors have already filed a notice to appeal the ruling, the outlet said, as Motgomery’s supporters begged President Trump to reduce her sentence to life.
Her attorney, Kelley Henry, said in a statement: “The Court was right to put a stop to Lisa Montgomery’s execution.
“As the court found, Mrs. Montgomery ‘made a strong showing’ of her current incompetence to be executed. Mrs. Montgomery has brain damage and severe mental illness that was exacerbated by the lifetime of sexual torture she suffered at the hands of caretakers.”
The attorney said the killer is “mentally deteriorating” and they are seeking to “prove her incompetence”.
Her execution could be one of the last, as President elect Joe Biden has denounced the death penalty publicly.
But it is not known whether he will halt federal executions when he takes office later this month.
Montgomery strangled her victim – who was eight months pregnant – with a piece of rope before retrieving the unborn baby girl with a kitchen knife.
They had met on the internet before the killer drove more than a hundred miles from her home in Kansas to Skidmore to meet her in person.
Prosecutors said she kidnapped the baby and tried to pass her off as her own.
Earlier this month, the US Court of Appeals ruled she can finally be executed.
The court concluded a previous judge was wrong to vacate her execution date.
Montgomery’s older half-sister Diane Mattingly, who was put into foster care aged eight, has called for her not to be executed.
She said Montgomery was abused by her mum Judy Shaughnessy and her boyfriends.
But those in Skidmore say she is a cold and calculating killer and have campaigned for her to be put to death.