The mother is a 15-year-old township resident.
She is going to be charged with recklessly endangering the welfare of a child.
Her parents were unaware of the situation, police say.
The baby girl was released from the hospital Thursday and placed into foster care.
According to police, the hours-old newborn baby girl was left abandoned on Tuesday.
Resident Terrell Phillips says the baby was found wrapped in a little white blanket on a porch.
Hours-old newborn baby found abandoned in Upper Darby, police say
"I picked it up, and as I picked it up the towel was unraveling and I saw the baby still had the umbilical cord on it," said Phillips.
The baby was left on the porch of Phillips' neighbor, Tom Dailey, who was just getting home from work.
"He said, 'Mr. Tom, what is this?,' and he says 'There's a baby here,'" recalled Dailey.