A baby boy is on life support in a Washington state hospital after a woman allegedly stabbed his mother to death and cut him from her womb.

Aracelo Camacho Gomez, 27, was 8-1/2 months pregnant when the attack happened on Friday night. She was found her with hands and feet tied together, having suffered "massive trauma" to her stomach area.

A 23-year-old woman of Laotian origin, Phiengchai Sisouvanh Synhavong, has been charged with her murder and is also accused of trying to pass the newborn baby off as her own.

The drama began when Kennewick Fire Department received a call from a woman claiming she had just given birth near a car park and thought the baby had died. Firefighters found a car with a baby in the back seat and a woman in the driver's seat naked from the waist down and holding an umbilical cord.

The woman allegedly repeated over and over: "Is my baby OK?"

"A fire captain pulled the baby from the driver's side of the rear door," Kennewick police chief Ken Hohenberg said.

The woman and baby were taken to hospital but medical tests showed the woman had not recently given birth. Soon afterwards police found Ms Gomez's body, with hands and feet bound, in Columbia Park, from where Ms Sisovanh had made her first emergency call.

Police searched Ms Sisouvanh's handbag and allegedly found gloves soaked in blood, bloody paper towels, latex gloves, a boxcutter, string, a baby bottle and baby socks, according to court documents.

According to the the victim's 10-year-old son and family friends, she was picked up with her children at a bus stop on Friday evening and given a lift to a department store by a woman who said that she had spare baby clothes for a boy. She gave the woman her address and telephone number.

The accused woman, who does not appear to have any criminal history, was married a month ago.

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