The baby's body was discovered by a member of the public in the car park of Morrisons, off the Black Country Route in Bilston, just after 9am on Sunday, April 11.
Police identified the mother and she was taken to hospital to receive appropriate care.
A 21-year-old woman and 36-year-old man were arrested at the time but later released on bail.
But West Midlands Police now says no further action will be taken over the incident as "no criminal offences have been identified".
A spokesman said: "A post mortem examination concluded that the baby was premature and sadly not viable.
"No criminal offences have been identified and no further action will be taken."
