Andrea Curry-Demus, 38, of Wilkinsburg, was charged with endangering the welfare of a child. She was charged 18 years ago in a plot to steal another woman's infant.
In a criminal complaint, police said they were called Thursday afternoon to West Penn Hospital after getting a report of a woman with a baby that wasn't hers.
Medics had transported the woman to the Pittsburgh hospital a few hours earlier after responding to a call that a woman with a baby needed assistance.
Tests at the hospital showed Curry-Demus wasn't the mother, police said.
Curry-Demus told police she befriended a pregnant woman and arranged to buy the baby after she miscarried in June and didn't want to upset her mother, according to the criminal complaint. Authorities said she told them she paid a woman named Tina $1,000 for the baby.
Curry-Demus said Tina came to her house on Wednesday with the newborn wrapped in a towel, police said. Curry-Demus said she called the medics Thursday because the baby was still dirty from birth.
Authorities said the baby's umbilical cord was attached when the newborn was taken to the hospital.
The baby will remain at the hospital until authorities determine who its mother is.
Authorities don't know where or how Curry-Demus got the infant, but are sure "she didn't give birth," Pittsburgh police Cmdr. Thomas Stangrecki said.
Pittsburgh police are investigating what happened at the hospital, while police in neighboring Wilkinsburg are investigating how Curry obtained the baby and whether its mother needs any help, Stangrecki said.
"I'm sure there is concern that there could be an injured woman somewhere or if there's a woman seeking treatment for her follow-up care after giving birth," Stangrecki said.
Wilkinsburg police Chief Ophelia Coleman did not immediately return a call for comment Friday.
Curry-Demus was arraigned Friday and jailed until she posts $10,000 bond and undergoes a psychiatric exam.
"I didn't do nothing," Curry-Demus told reporters as she was put into a police car Thursday.
Curry-Demus pleaded guilty in 1991 to aggravated assault after stabbing a Wilkinsburg woman in an alleged plot to steal the woman's infant. Curry-Demus, then known as Andrea Curry, was sentenced to 10 years' probation.