Ambulance paramedics treated the woman at the scene before she was taken to Sydney's Westmead Hospital in a critical and unstable condition but she and her baby were unable to be saved.
Emergency services responded to reports of a pedestrian struck by a vehicle at a car park entrance at Hornsby in the city's northwest ábout 8pm on Friday.
A 19-year-old man behind the wheel of a BMW and a 48-year-old male driver of a Kia station wagon were uninjured.
Both were taken to Hornsby Hospital for mandatory testing.
No other occupants were in the vehicles.
Police are expected to be allege that the Kia was struck from behind by the BMW causing it to crash into the woman.
The younger driver was arrested at a home in Wahroonga in the early hours of Saturday and questioned by detectives.
He's been charged with dangerous driving occasioning death including causing the loss of a foetus and refused bail to appear before Parramatta Local Court on Saturday.
Anyone with information about the crash or who was in the area at the time and recorded dashcam or mobile phone footage is urged to come forward to police.
A critical incident investigation was also under way on Saturday after the death of a motorcyclist who was being pursued by highway patrol officers, at inner city Zetland.
An attempt was made to stop the bike on Southern Cross Drive shortly before midnight on Friday, with the chase activated after the rider refused to pull over.
Police say the motorcycle crashed into the rear of a van in front, a short time later.
The rider was treated by police before paramedics arrived but died at the scene.
A critical incident investigation team will examine the circumstances surrounding the incident, which will be the subject of a review by the force's Professional Standards Command and overseen by the states Law Enforcement Conduct Commission.
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