Police say 31-year-old Jeremy Smith injured the baby during an altercation with the child's grandmother.
Police say Smith has been in Somerset County Jail since Jan. 1 for allegedly assaulting the woman.
But the infant's injuries were unknown until Wednesday when it was treated in Pittsburgh's Children Hospital for an unrelated medical problem.
Police accuse Smith of injuring the baby by assaulting the grandmother while she was holding the infant.
