Police said the accident happened at 7:45am on the outbound Ploenchit expressway between the Din Daeng and Klong Toei exits.
Police said the third car in the middle caught fire and a passenger, Tharinee Rani, who is six-months pregnant, had to be rushed to the Rajvithi Hospital. She escaped the car before it caught fire.
Police and rescuers managed to extinguish the fire quickly. The vehicles were cleared from the expressway and traffic returned to normal by 8:30am.