Louise Woodward, 19, from Chester, was arrested last month after allegedly shaking nine-month-old Matthew Eappen and fracturing his skull at the baby's home in Massachusetts. Miss Woodward had lived there as a nanny since November.
A medical examiner said the baby died of a blunt-trauma wound to the head, caused by his head striking a hard surface.
He was taken to hospital on 5 February and died four days later.
A grand jury in Middlesex County, Massachusetts, returned a first-degree murder charge yesterday against Miss Woodward, who has been held without bail since her arrest.
The teenager has already been indicted on a charge of assault and battery over the baby's death, to which she pleaded not guilty. The Middlesex County District Attorney's office said they were expecting her to enter a plea on the murder charge in court tomorrow. If convicted, she would face a mandatory life sentence in state prison with no eligibility for parole.
Miss Woodward went to work in the United States last year.Reuse content