Police are searching for the gunman.
The 11-month-old boy was in the back seat of the car when he was shot in the back of the head, in the chest and twice in the backside.
Police said the shots were fired at 7th and West Luzerne streets, but the woman kept driving to the 4900 block of Camac Street in the Fern Rock section of the city. It was there where she realized the baby was hit.
According to authorities, both were initially taken to Einstein Hospital. The baby was later taken to St. Christopher's Hospital for Children. It's not clear if the stepmother was wounded.
Police have asked anyone with information about the shooting to call them.