The incident happened at around 1am after she dropped her partner off at work.
Her two-year-old child suffered minor injuries, and her 32-week-old unborn child was later delivered in hospital.
Police say the driver of a stolen Toyata RAV 4, with three passengers, left the scene immediately following the crash but has since been found.
All four, aged between 12 and 15, are now helping police with their inquiries.
Police had earlier attempted to intercept the stolen car, but it drove off.
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Tasmanian Police Commander Glenn Frame says they did not pursue the car in accordance with Tasmanian police policy.
"Our sympathy and condolences go out to the family and the friends of the deceased in this case. It's a tragedy that was an avoidable tragedy. This is a terrible situation where stolen vehicles are allegedly driven in a manner that puts the public at risk."
