She discovered her child alive in a drawer in the morgue's refrigerated room, where the child had been for 12 hours.
The mother tells Argentinian TV she thought she was hallucinating when she heard a whimper and saw signs of life. Doctors say the girl is now in good condition.
The mother says the infant was born three months prematurely and the hospital said she had died of unknown causes.
Health officials say they are investigating those involved in the incident.