Serena Williams' rep is finally confirming -- after an oddly long delay -- the tennis superstar and her fiance, Alexis Ohanian, are expecting their first child.

No shocker here ... our sources told us it was true when Serena posted the shot of herself with the caption, "20 weeks." Her rep just released this statement: "I'm happy to confirm Serena is expecting a baby this Fall."

Here's the thing ... it's been about 7 hours since Serena let the cat out of the bag on Snapchat.

Hell, she might be 21 weeks pregnant by now! In any event ... mazel tov.