Sources close to the couple tell TMZ ... Offset's never personally met Celina Powell, who posted a picture of a sonogram Friday claiming she's having a baby girl ... and Offset's the baby daddy.

She captioned the post, "I hope @offsetyrn & I work this out for the sake of Karma Lee Cephus."

It's allegedly not the first time Powell's claimed other rappers as her baby daddy ... we're told her past accusations have included Fetty Wap and Chief Keef. She cops to the prior allegations too ... writing in her new post, "I know y'all like 'this bitch done faked hella pregnancies.'"

While Offset claims he doesn't know Celina personally, the two have had contact in the past -- a video making rounds online shows the two during a FaceTime call -- but our sources say that was only Celina prank calling the Migos rapper after she somehow got his personal number.