Officers responded to what is believed to be a rooming house.
Police said a 28-year-old woman was shot in the face. She was lying on the floor, bleeding heavily. Officers also found a 6-month-old boy lying on a bed in the room.
The woman was transported to Lankenau Hospital in Philadelphia where she later died.
Police interviewed fellow residents of the rooming house.
"They heard an argument between the victim and what they believe was a male voice. They heard one gunshot, then they heard someone run down the steps," according to Chief Inspector Scott Small.
When one of the witnesses looked out the window, they told police they thought they saw someone running north on 55th Street.
Police said the infant was not physically injured in the incident.
No arrests have been made.
Anyone with any information is asked to call police at 215-686-TIPS.