The teenage parents were on the run after taking their three-day-old baby girl from a western Sydney hospital and a wide-scale campaign was launched to find them.

Police said the three were found safe and well about 4.15pm on Thursday at Willmot.

There were concerns for the welfare of the three children after the young parents didn't formally discharge from Nepean Hospital when they walked out in the early hours of Thursday.

Fifteen-year-old mother Jenifer Morrison and 14-year-old father Jayden Lavender were believed to be with an unknown man with whom they left the hospital in the dead of the night.

The police said they were speaking to the teenagers and ambulance officers were called to assess the infant as a precaution.

Nepean Hospital said it had immediately notified police when it was discovered the mother and child had not been discharged.

Police had issued a direct plea to the young parents, saying they were not in trouble and urging them to contact police for the safety of the baby and the mother.

"To Jenifer and Jayden, the two parents, you're not in trouble. We'd just really appreciate if you could go to the hospital and check out to see if you're alright," Penrith detective inspector Grant Healey told reporters in Sydney.

"The doctors still need to see your baby and make sure it's progressing nicely."