Aaron Fitzpatrick, 19, of the 700 block of Tree Street in South Philadelphia, will be charged with murdering 24-year-old Tiffany Gillespie.
Police say Fitzpatrick was Gillespie's boyfriend. They say Gillespie was arguing with Fitzpatrick early Thursday morning in the basement of her home at 2327 Mildred Street while her mother and two young children were upstairs.
Police say Fitzpatrick was upset that Gillespie had become pregnant with his baby.
They say in the course of the argument, Fitzpatrick pulled out a gun and shot Gillespie once in the head, killing her.
Fitzpatrick fled the scene as Gillespie's mother called 911.
Police say Fitzpatrick was picked up for questioning several hours later, questioned and then arrested. He was in custody awaiting murder charges Thursday night.