The teenager was a passenger in a red Peugeot 109 along the A48 between Mathern and Chepstow when it left the road and crashed into a wall.
Heavy rescue equipment had to be used by fire crews from Chepstow and Malpas to release both the girl and the female driver from the car.
Both women were taken to the Royal Gwent Hospital with serious injuries.
Police are continuing their investigations into the incident. They are also appealing for any witnesses to the scene to help them with their enquiries.
*A replacement vehicle can usually be supplied within 4-6 working hours, where there is an admission of liability from the other person’s insurance company. If this person is not known, their insurance details are not known, they are uninsured or there is a potential dispute over who caused the accident, a replacement vehicle cannot be provided.
