It depends on many factors, including if they’re hit while walking or while in a car, how far along they are, how serious the mother’s injuries and where they are, and the health of the mother and baby to that point.
Labor can be triggered through trauma in a number of ways, and whether or not woman or child survives depends on a number of factors including age, health, severity of injuries, how quickly they get medical help, and more.
As far as issues for the baby, again it depends on how premature it is, how quickly medical attention is given, and what injuries are sustain as a result of the accident.
Doing some research on premature birth, car accident injurires, and pregnancy in general for a woman the age of your protagonist should help give more background and help you decide what happens.
Hope this helps!