The teenager who killed Hobart pregnant mother Sarah Paino when he crashed a stolen four-wheel drive into her car during a joyride has been sentenced to five years in detention.

Key points: Judge finds teen had "reckless disregard for life"

Judge finds teen had "reckless disregard for life" She handed down a "heavy sentence" for "chillingly dangerous driving"

She handed down a "heavy sentence" for "chillingly dangerous driving" Youth remained emotionless during sentencing

The 16-year-old was sentenced as an adult after pleading guilty to Ms Paino's manslaughter in May, and will have to serve a minimum of two-and-a-half years before being eligible for parole.

Justice Helen Wood said she had handed down a "heavy sentence" and described the teenager's driving as "chillingly dangerous".

Manslaughter under the Criminal Code carries a maximum adult sentence of 21 years in jail.

The maximum sentence for the same offence under the Youth Justice Act would have been two years in detention.

As the boy was led away in Hobart's Supreme Court, the youth's family cried and shouted:

"Bye baby, you're doing great."

The youth, who cannot be named for legal reasons, remained emotionless throughout the hearing, and wore a suit jacket and shirt.

The teenager had driven at high speed through central Hobart in a stolen four-wheel drive before running a red light, slamming into Ms Paino's car in January.

At the time of the fatal crash, the stolen car was travelling at an estimated 110 kilometres per hour with no headlights.

Ms Paino suffered catastrophic injuries and did not survive, but paramedics kept her alive until her 32-week-old unborn son was delivered a short time later at the Royal Hobart Hospital.

Her two-year-old son was sitting in the back seat of the car and also survived.

'Very little mitigates the seriousness of the crime'

Justice Wood said the family's suffering and grief was "immeasurable".

"They were a happy family and she was a devoted and loving mother," she said.

Pregnant Hobart woman Sarah Paino's car after fatal crash in Davey Street, Hobart. ( ABC News )

She said at the time of the crash the youth showed very little remorse, but had since expressed regret.

"He is remorseful and has a level of understanding of the consequences," Justice Wood said.

"He has thought about what he could have done to avoid the crash."

But Justice Wood said there was very little that mitigated the seriousness of the crime.

The teenager's attention deficit hyperactivity disorder (ADHD) and intellectual deficits were relevant in mitigation in reducing his culpability to a degree.

But she said the nature of the crime revealed a "reckless disregard for life and deliberate-risk-taking".

Ms Paino's family and friends were at the court but declined to make any comments, leaving through a side entrance to the court.

Stolen car was travelling 'insanely fast' before crash

Police at the scene of the crash in January. ( ABC News: Ros Lehman )

The tragedy unfolded after Ms Paino had dropped her partner and the children's father, Daniel Stirling, off at work at a city bakery.

Her car was struck on the corner of Argyle and Davey streets.

Witnesses told police they had seen the stolen car travelling "insanely fast" and seemingly out of control before the crash, swerving across the road.

The court heard the teenager had switched the headlights off to evade police and had been travelling on some of Hobart's main roads at speeds of up to 200 kilometres an hour.

Prosecutors had called for the boy to be sentenced as an adult on the grounds of the seriousness of his conduct and his reckless driving before the fatal impact.

The sentence had been adjourned for more than a month to allow for further psychiatric evaluations of the teenager.

Prosecutors told the court it showed the boy was not mentally impaired enough not to understand the gravity of his offending.