McCarthy Brady, who had been living illegally in Quincy, Massachusetts and married last year, was remanded in custody charged with violently assaulting the one-year-old girl.
The baby suffered brain damage and died in hospital two days after her first birthday.
McCarthy Brady was warned in court that a charge of assault and battery causing substantial bodily injury may be revised to murder depending on the Chief Medical Examiner's report.
Middlesex District Attorney Gerry Leone said: "This is an extremely troubling case."
McCarthy Brady worked as a nanny for 13 years. She was employed for the last six months by entrepreneur Sameer Sabir, the baby's father and his wife Saddiqui, who is originally from Karachi, Pakistan. The couple moved from London to the US.