The baby suffered multiple injuries – including bruising, a skull fracture and multiple rib fractures – and had THC, the active ingredient in cannabis, in his system when he died.
His death was found to be due to chest trauma, which caused the heart to go into an abnormal rhythm and cause the brain to swell.
Today Justice van Bohemen found the woman guilty on one charge of assaulting the boy with a shower head and one of ill treatment by failing to protect her child from harm or injury.
She was remanded on bail to be sentenced in the court on April 23.
Name suppression for the mother lapsed when she appeared for sentencing, but she cannot be named, at this stage, as the boy's father has name suppression after he had earlier been dealt with by the courts. The details of that have been suppressed also.
A media application to lift name suppression for the father has been filed, but has yet to be heard.
During the trial Crown Prosecutor Mike Smith said while the mother did not blow cannabis on the baby's face, she was nonetheless a party to the act by allowing it to occur when the child was in her care.
He said the Crown was not required to prove the offence was committed by her or someone else, but that it happened in her care which was a departure from the standard of care expected of her.
Smith said she minimised her offending and shifted the blame on to others.
Defence lawyer Wayne McKean said an analysis of evidence showed it was her partner who was violent towards the baby, that he was a large man, and was a possible cause of unexplained injuries on the child.
The baby was healthy and well looked-after when checked by Plunket at 3 months old, he said.
McKean said the bruising on the baby's forehead was likely to have been caused by knocking on his head with knuckles and the injury on his ribs could have been as a result of punching and squeezing him - acts her partner did.
The fractured skull was caused by him hitting his head against an object such as a game console or something protruding from a wall with a flat edge.
On the thumb fracture, McKean said the pathologist didn't see it and the Crown did not call the radiologist who analysed x-ray results.
Her partner allegedly blew cannabis over the baby's face and gave him alcohol but it wasn't clear how much was given, he submitted.
The mother told the court she would take her baby away from her partner when the latter become "rough" and the 5-month-old would cry.
She saw her partner squeezing the baby with his knuckles on both sides of his ribs and on top of his head, and slap him on his forehead and mainly around the back of his head.
On one occasion, she said the baby's father allegedly held the baby then dropped him on to the carpet in the lounge which resulted in the child fainting.
However, Justice van Bohemen found the woman not guilty of supplying the child cannabis and alcohol or that she had committed an assault other than with the shower head.
The judge said he was satisfied beyond reasonable doubt that the mother assaulted the baby with a shower head, even though her earlier admission was retracted.
But, he said, the action did not cause the baby's death, and she did not intend to hurt the infant.
"I've considered the injuries suffered by [the baby], in particular the fracture to the skull and many fractures to the ribs. You knew he was hurt, even if you did not know the nature of the injuries," van Bohemen said today.
"You knew that he had been squeezed and had knuckles to his ribs to the point that you heard audible cracks.
"You did not seek care. That was a major departure from the standard of care expected and that was likely to cause [the baby] suffering or adverse effects."
