The woman was shot in the left leg, police said. She was taken to the Hospital of the University of Pennsylvania.
The gunman was described as black, 25 to 30 years old, weighing about 210 pounds and standing about six feet tall, police said. He was wearing a white T-shirt, blue jeans, and a fitted baseball cap. He was last seen running south on Bucknell.
Police spokesman Lt. Raymond Evers said a fight among girls from South Philadelphia High School led to another altercation and, ultimately, the shooting.
Girls involved in the fight showed up at a 12-year-old girl's house on Bucknell, Evers said. "The girls were pushing into the house," Evers said.
Some neighbors got involved, and one girl called an unidentified man, Evers said. A group of men arrived at the scene and the fighting escalated. Two men pulled out guns and one of them began firing.
Chief Inspector Scott Small said the two children were in the house when they were wounded. Blood was visible on the living room floor just inside the front door. The woman was in front of the house when she was hit, Small said.
Police Commissioner Charles H. Ramsey also responded to scene and denounced a culture of violence among young people.
A neighbor said he saw "a pack of kids raising hell" down the block. Soon afterward, he heard gunshots. The neighbor said he called 911 and by the time he got off the phone, a patrol car had arrived. By that point, everyone involved in the melee had scattered.
The officer "ran through the house and he came out with the baby" and sped off, the neighbor said.
Contact staff writer Robert Moran at 215-854-5983 or email@example.com.