Jannette Garlow, 23, was counting out $2,000 (around £1,600) to give to a cashier at a shop in Los Angeles when a man tried to brazenly snatch it and run.
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Jannette, who is eight months pregnant, turned and grabbed the attacker.
The robber slipped on rug in the shop dropping some of the cash as he grappled with Jannette.
Incredible CCTV footage show how Janette then gave chase as he fled the shop to a waiting car.
KTLA 5 News
KTLA 5 News
In the end, the cowardly thief only got away with $40 (about £32).
Janette was about to exchange the cash for a money order to pay her rent.
She said "instinct" kicked in at first.
"All I could think was, my husband just gave me all this money for our rent, and we don't have more to replace it. I needed to save his hard-earned money," she told the New York Daily News.
Janette said it wasn't until she had hold of the getaway car door that she realised she should stop.
"I let go of the car because no money is worth risking that," she added.
The mum-to-be went to hospital as a precaution and fortunately both Janette and her baby boy were uninjured.
LA County police are investigating.