The baby was born this month without Covid-19 but with the virus antibodies, the Straits Times newspaper reported on Sunday, citing the mother.
"My doctor suspects I have transferred my Covid-19 antibodies to him during my pregnancy," Celine Ng-Chan told the paper.
Ng-Chan had been mildly ill from the disease and was discharged from hospital after two-and-a-half weeks, the Straits Times said.
Ng-Chan and the National University Hospital (NUH), where she gave birth, did not immediately respond to a request for comment.