A supreme court judge has overlooked the "inadequate" Youth Justice Act's maximum penalty to sentence a 16-year-old boy over the manslaughter of a pregnant Hobart woman.
Justice Helen Wood on Tuesday sentenced the juvenile under adult law to serve at least two-and-half years in custody as part of a maximum five-year term.
In January, the then-15-year-old ran a red light in a stolen car at a speed of almost 120km/h, T-boning a hatchback driven by 24-year-old Sarah Paino who was 33-weeks pregnant.
She was killed instantly but her baby was delivered minutes later in hospital, while her two-year-old son in the back seat avoided injury.
"A lesser sentence would not adequately reflect the gravity of the defendant's conduct," the judge said.
"He chose to drive unlicensed, and to drive dangerously in the manner he did - he took on an adult responsibility and he created danger for others."
If sentenced as a child, the boy - who cannot be named due to his age - would have faced a maximum of two years detention.
"I consider that penalty is plainly inadequate," Justice Wood said of the Youth Justice Act.
During the lengthy sentencing Justice Wood detailed how the teen drove at speeds up to 160km/h on the night he killed Ms Paino and fled the scene when his joy ride came to a chilling end.
"The defendant drove into the intersection, disobeying a red traffic light and crashed directly into the driver's side of the (hatchback) with catastrophic force and tragic consequences.
"His actions killed Ms Paino, nearly killed her unborn child and exposed her two-year-old son to the risk of death or grave injury."
Victim impact statements from Ms Paino's partner and family detailed feelings of immeasurable grief.
Justice Wood said the boy must be given the chance to reform.
The court was told of his troubled past which included learning difficulties and an ADHD diagnosis along with the use of cannabis and alcohol from age 12 and in more recent times, the drug ice - although he was not intoxicated on the night of the crash.
His attendance at school has been poor and he has a lengthy criminal history including offences of assault, assaulting police, stealing, stealing cars and aggravated burglary.
"The defendant has spent his late childhood and early adolescence in the company of an antisocial and substance-abusing peer group rather than being socialised within a family and school environment," Justice Wood said, also noting he had very few limits placed on him.
About 10 relatives and friends of the boy gathered in court to hear the sentence.
A woman called out "bye, baby" as he was led away.
Ms Paino's partner Daniel Stirling left the court without talking to reporters.
The sentence closes a case which attracted national attention after two young boys were left without a mother.