A one-month-old baby in the US has died of starvation in a parked car while her mother and stepfather dined at an all-you-can-eat buffet restaurant, police say.

Betsey Kee Stephens weighed just 1.8kg when she died last week, more than a kilogram less than her birth weight and little more than half what she should have weighed at that age.

Police in Florida charged mother Ruby, 23, and stepfather Roy, 48, with the murder of the 22-day-old on Monday, a week after her death.

"In my opinion this child suffered from day one of birth, to the time that she passed away," Assistant Police Chief Mike Link said in a press conference overnight.

"When I saw the photograph — it shook me to my core because in thirty years, I have never seen anything like that."

Medical examiners ruled Betsey died three-and-a-half hours before her parents called emergency services.

The couple had kept the baby and her two older siblings in the car while they went to eat at Golden Corral, a chain restaurant known for its all-you-can-eat buffet.

When they arrived back at the car, they noticed Betsey's feet were unusually cold, prompting the emergency call.

Both her mother and stepfather claim they fed they baby several times during their car trip from Indiana to Florida, but medical examiners say that is not possible.

Betsey's siblings, aged one and two, are in good health and are now in the care of the state.

""If you don't want the child, give it up for adoption, give it to somebody," Assistant Chief Link said.