The victim was already in a "state of decomposition" by the time officers and fire personnel arrived, WBNS-TV reports.
The mother, Melissa Thorp, was reportedly the one who called police about her deceased son.
The coroner's office determined that the 4-month-old's skull was crushed between two objects, according to court records obtained by WBNS. On Sept. 13, police said the mother was arrested and charged with murder.
Thorp remains held on $750,000 bond and is scheduled for a preliminary hearing on Sept. 23.