Tyrese Lynch, 33, surrendered to Philadelphia police around noon Monday for the killing of Isis Williams.
Authorities found Williams dead around 4 a.m. Friday inside a home on the 6300 block of North Woodstock Street in Logan.
Police believe Lynch shot Williams in the head with her 5-day-old daughter in a crib just steps away.
Williams' aunt, Aspara Agyeman, said Williams was on the phone with her mother when gunshots rang out.
"Her mother heard the shots," said Agyeman.
The mother of three had given birth just days before. The baby girl, now 1-week-old, is staying with Agyeman.
"Even though I may be the one that ends up raising her, she's gonna know that Isis is her mother and that she was loved," she said.
On a GoFundMe page set up to help with funeral costs, a family member says the relationship between Williams and Lynch was abusive.
Investigators said there were signs of a struggle in the home and furniture was overturned, but the baby was not injured.
Neighbors said they heard an argument before Williams was found.
"He was calling her derogatory names," said neighbor Melvina Hall. "It was very loud."
A vigil is planned for Monday evening to remember Williams in her neighborhood.