Sarah Paino, 24, died almost instantly after suffering severe neck and chest injuries in the central Hobart smash at about 1am on January 22, but when rescue crews realised she was pregnant they continued CPR until her unborn son was delivered in hospital a short time later.

The youth, now 16 and unable to be named due to his age, on Thursday pleaded guilty to manslaughter in Tasmania's Supreme Court, admitting he reached speeds in excess of 200km/h in the lead-up to the crash.

"This was just bound to happen," Director of Public Prosecutions Daryl Coates SC said of the smash.

"It may not have been Ms Paino coming through the intersection but the conduct the accused entered into was highly likely to cause an accident where death was going to result."

The boy's prolonged risky behaviour on the night means he should not be tried as a child which would see him receive a maximum possible penalty of two years detention, Mr Coates said.

"You should exercise jurisdiction under the Criminal Code and Sentencing Act," he told Justice Helen Wood.

"It is my submission that (the Youth Justice Act) does not come in any way commensurate with the offence that has been committed."

A witness described seeing the stolen Rav 4 travelling "insanely fast" just seconds before it ploughed into Ms Paino's Nissan hatchback.

She had just dropped her partner of eight years, Daniel Stirling, at work and was returning home with their two-year-old son in the back seat when the smash happened.

Crash investigators have calculated Ms Paino's car was travelling at 17km/h when it was struck by the Rav 4 which was doing at least 110km/h.

The impact sent the Nissan into a spin before stopping 36m away with a crying but uninjured toddler restrained in the back.

Barrister Kim Baumeler told the court her client later learned of Ms Paino's death on television news.

"The worst part is knowing that his actions have resulted in two children growing up without a mother," Ms Baumeler said on behalf of the teen, who she added is remorseful.

Victim impact statements from Mr Stirling, and Ms Paino's sister, mother and father Michael Paino were each read to the court, with the latter stating he is constantly reminded of the "careless and sickeningly violent" circumstances of his daughter's death.

"I've suffered in every way imaginable since the day Sarah was taken from us," Mr Paino said.

Baby Caleb is fine but "we don't know long term", the grandfather added outside court.

Justice Wood will sentence the teenager on June 22.