Baby Hunter White died due to a lack of oxygen and blood flow to the brain because he could not breathe or move when his dad fell asleep.
A jury at Nottingham Crown Court heard the tragic 18-day-old tot passed away as a result of what is called "positional asphyxia" when he was in the care of his father, Adam White.
White, 45, of Foxhill Road East, Carlton, "fell asleep under the influence of something" and Hunter, facedown, died.
Mum Terri Walters, 32, of Kingsway Gardens, Hucknall, was in the house at the time and had also been taking drugs and went to bed, leaving the baby with his dad when "not safe to do," explained Judge Gregory Dickinson QC, the Recorder of Nottingham.
Both parents were charged with cruelty to a child which they have now admitted - Walters entering her plea this morning, March 2, and Adam White pleading guilty last September.
Hunter was pronounced dead by paramedics who were called to Kingsway Gardens, Hucknall, on July 10, 2020.
Judge Dickinson apologised to the jury and explained what had been going on when they returned to courtroom three this morning.
He described how Walters had not been in the dock, where a defendant normally is, on Monday, February 28.
"On Monday the dock was empty," he said. "The defendant, very, very distressed, left the court building, and it was necessary to issue a warrant for her arrest. She was brought back to court this morning and spent time with her very experienced and very good barrister and solicitor.
"Now she has pleaded guilty. This case concerns the death of a three-week-old baby. The baby died as a result of what is called 'positional asphyxia'. The baby was facedown and could not breathe or move and died of asphyxia. This defendant and her partner were mother and father.
"They both have longstanding drug problems. The baby was in the care of father. He fell asleep under the influence of something. The baby, facedown, died.
"The mother was in the house, also taking drugs, went to bed, leaving the baby with the father when not safe to do."
After disappearing from the court building before her case started, a warrant was issued for Walter's arrest.
When she was found by police, she was held in custody and brought before the court where she decided to change her plea to guilty to cruelty to a person under the age of 16.
Her voice cracked as she said the words "guilty" from the dock where she appeared looking unkempt and wearing blue jeans and a patterned shirt.
The charge read out to her said that the offence was on July 10, 2020, and although having responsibility for Hunter White she had wilfully neglected him.
Walter's barrister, Sue Rodham, explained there was a draft basis of plea - where a defendant pleads guilty on a particular basis - and she applied for a pre-sentence report in the case.
The parents of Hunter will be sentenced together on April 22 in the afternoon. Judge Dickinson reserved the case to himself. He has allowed one-and-a-half hours.
He granted bail to Walters, acknowledging it was a serious and sensitive case, and there are "considerable emotional pressures on the defendant".
But he said that must not be taken as an indication of the likely sentence - as the charge does pass the custody threshold but he will decide.
He advised her to stay in close contact with her solicitors, "who are doing the very best to help you", and he added a condition of residence, so she must spend every night at home and cooperate with the probation service as they put together her report.
"You will be back here for sentencing with Mr White and then, only then, will I decide what the sentence must be," said the judge.
When Walters was approached by Nottinghamshire Live for comment as she left the court building, she said "no".
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