There were warning signs a teenager's violence was escalating before he fatally bashed his baby son at a regional West Australian hospital, a coroner has found.
The 15-year-old was a ward of the state when he inflicted severe head injuries to the infant at Bunbury Regional Hospital in February 2014.
He was also subject to a conditional release order over a February 2013 incident at a train station where he threw a knife at the mother of his child and it hit a bystander.
The West Australian Coroners Court heard he had a dysfunctional upbringing, had begun using drugs at age 11 and dropped out of school in Year 8.
His relationship with his 16-year-old girlfriend was highly volatile and suspected of being marred by domestic violence.
Due to these issues, various government agencies were involved in pre-birth planning with the couple but the birth came six weeks early, so there was less time to plan for longer term concerns.
Despite reports the teen had been aggressive and violent towards his partner and/or mother at the hospital a fortnight before the child's death, the Department for Child Protection decided there was insufficient evidence to remove the baby from his parents' care.
In another incident, a hospital midwife observed the teen looking irritated and annoyed before he tried to feed the child, prompting distressed crying and causing a small cut on his gum.
She told the teen he needed to be gentler when feeding the infant, but didn't write the incident down as she felt he'd listened to her.
Two days later, when he was left alone with the baby for just a few minutes, he deliberately struck the child's head against a hard surface, fracturing his skull and inflicting brain injuries.
Coroner Sarah Linton said in inquest findings released on Tuesday it was difficult to see how anyone could have predicted the extreme violence that came two days after the feeding incident.
"There was arguably not a sufficient basis for the department to take action based upon what was known at the time," she said.
"Nevertheless, there were warning signs that were not properly heeded by those involved."
That was largely due to a lack of real understanding and knowledge about the teen, his increasing violence and lack of ability to regulate his emotions, she said.
After he pleaded guilty to manslaughter and was sentenced to 10 years in detention, he was diagnosed with fetal alcohol spectrum disorder, which led to him successfully appealing the sentence.
The term was reduced to seven years, with eligibility for supervised release after serving half the term.
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