A US woman has admitted to mutilating her three-year-old son’s genitals and starving him so badly his rib cage was visible and his backside was just sagging skin.

Brittany Nicole Lippincott, 29, from Fairmont, West Virginia, pleaded guilty to one count of child neglect resulting in injury and one felony count of child neglect creating risk of injury or death, the West Virginian Times reports.

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Lippincott, who was also wanted on drugs charges, was arrested after her three-year-old son was found at their home locked inside a room that had been bolted shut during a welfare check in May.

The boy was found in the room dressed only in his underwear.

The police officer who found him said he looked, “emaciated, pale, with apparent bruising on his face near the eye and mouth", according to court documents.

Lippincott is awaiting sentencing. Credit: West Virginia regional jail authority

'It's not my turn'

The boy was taken to hospital for treatment where officials said the outline of his rib cage was visible from both sides of his body and he weighed only 10 kilograms.

It was also found that he had not formed a backside, but instead only had sagging skin, because of the emaciation.

When asked about the injuries to his genitals, the boy said: “Mummy did it with pliers”.

The boy also told officials he would “sit at the table and watch them eat, its not my turn”.

Police seized drug paraphernalia from the house.

'Neglectful parent'

Judge Patrick Wilson accepted the guilty plea and told the court that Lippincott is “an abusive and/or neglectful parent”, according to the West Virginian Times.

Lippincott, who told the court she’d “medically neglected” her son, will have to register in the state’s child abuse and neglect registry.

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She also later admitted to neglecting his nutritional needs.

Prosecutor Jennifer Pigott said since being removed from his mother’s care, the boy is now “thriving”.

The date for Lipincott’s sentencing had not yet been set.

There was initial confusion over the spelling of the boy's mother's last name, which was written as "Lippencott" on the court schedule but confirmed to be "Lippincott" by the Marion County Sheriff's Office.