A 21-year-old woman has been found guilty of the new charge of familial homicide by failing to prevent her baby son's murder by her partner.
Rebecca Lewis's 13-month-old son Aaron Gilbert died from brain damage in May 2005. He had suffered 50 injuries.
Her partner at the time, Andrew Lloyd, 23, pleaded guilty to murder at Swansea Crown Court. The pair are due to be sentenced together at a later date.
Lewis denied familial homicide. She is one of the first to face the charge.
As the verdict was delivered, she broke down in the dock and held her head in her hands.
Four weeks before at the start of the trial, the prosecution alleged that although Lewis was largely absent when the violence was inflicted on her son, her silence allowed it to continue.
Paul Lewis QC, prosecuting, said Lloyd had caused a "constellation of some 50 injuries" to Aaron's body.
Lloyd pleaded guilty when presented with medical evidence
"Andrew Lloyd systematically and violently assaulted a defenceless child and ultimately killed it," he added.
"Aaron's mother, Rebecca Lewis, who should have protected and cared for her son, allowed the injuries and ill-treatment to continue until, eventually, her inaction cost Aaron his life."
The jury heard Ms Lewis had begun a relationship with Lloyd following the break-up of her relationship with Aaron's father, Gareth Gilbert.
Lewis told the trial that Lloyd had initially been "brilliant" with the baby but that within weeks of him moving in problems emerged. Eight weeks later Aaron was dead.
The court heard that Lloyd declared he hated the child and threatened "to smash your little head in".
Lewis acknowledged that the baby became terrified of Lloyd. She had seen Lloyd flicking his ears and feet when he cried, picking him up by his ears and ankles and throwing him onto a bed and settee.
But, she told the court, in spite of that she had trusted Lloyd.
On the day before Aaron died on 5 May 2005, Lewis left the baby with Lloyd for about 15 minutes to go shopping. When she returned, Aaron was lying on the kitchen floor. He was grey. Lloyd was bent over him trying to administer mouth-to-mouth.
Aaron Gilbert died of brain damage with 50 injuries on his body
Doctors at Swansea's Singleton Hospital and later the University of Wales Hospital, Cardiff, tried for 18 hours to save Aaron's life but he died of brain damage.
Lewis admitted in court she had let Lloyd mistreat the baby because she was frightened as he had threatened to kill her if she left him. But, she told the jury, she did not think he would hurt Aaron "seriously".
The court also heard that neighbours began to notice bruises and injuries on the baby.
Lewis' cousin Sharon Hurlow made two anonymous calls to Swansea social services, who wrote to Ms Lewis.
She was asked to attend an appointment. It turned out it was booked for the day Aaron was taken to hospital, fatally injured.
However, the court heard, the letter had been sent to the wrong address.
The charge of familial homicide came into force in May 2005 under Section 5 of the Domestic Violence, Crimes and Victims Act 2004.
The Act allows any person in a household to be prosecuted for failing to intervene to protect a child victim of domestic crime.
Lewis denied allowing her son's murder and said she had thought about leaving Lloyd but was afraid of what he might do.
"He said if I left he would kill me or my ex-boyfriend," she said
The jury also heard her explain that she loved Lloyd and had trusted him, and did not believe he had murdered Aaron until she heard him admit it in court.
Lloyd initially denied murder but later wept uncontrollably when he changed his plea after being presented with medical evidence.
Judge Mr Justice Langstaff told Lewis a prison sentence looked "inevitable".