The woman, in her 30s and who is believed to be Sri Lankan, had tried to stop an attempted fraud at the shop where she works in Leigh-on-Sea, Essex.
Police said she was struck by a grey Volkswagen Golf and suffered injuries to her head and face and internal injuries.
The victim, who is understood to be four or five months pregnant, was taken to hospital in a stable but life-threatening condition after the incident at around 5pm on Monday.
She was said to have been on her own in the off-licence, in London Road, when the incident happened.
An Essex Police spokesman said: “It is not believed the baby was harmed.”
A 25-year-old man has been arrested on suspicion of dangerous driving, assault, fraud by false representation and failing to stop at the scene of an accident.
He was detained at an address in London on Tuesday and taken to Southend police station, where he remains in custody. A car has also been recovered.
A friend, who did not want to be named, said: “She was on her own at the time and she is four or five months pregnant. It’s really sad, she only came over from Sri Lanka a year ago”
Locals reacted in shock to the news that the family-run business was targeted.
Tom Triggs, 54, said: “She has a little boy about two and they are a lovely family. My grandchildren go in there all the time – they are really nice people.”
John Lloyd, a barber who works opposite the shop, said: “It’s scary, round here you wouldn’t expect it. I can’t get my head around it.”
Detective Inspector Stuart Truss said: “The shop was very busy at that time of day and there were also a lot of people in the area who may have seen what happened.
“I would urge anyone who witnessed the incident or has any other information about it to come forward and contact us.”