'Southern Charm' Star -- I Gave Birth (For Real) To My Co-Star's Baby

So much for reality television ... the 21-year-old star of "Southern Charm" who had a pregnancy scare on the show -- but said her test came back negative -- just had a baby.

Sources close to the show tell us Kathryn Dennis gave birth to a healthy baby girl 2 weeks ago -- the very day the pregnancy-scare episode aired on Bravo. And the baby daddy is her mega-rich 50-year-old co-star Thomas Ravenel.

The hookup between Kathryn and Ravenel was one of the big storylines on the show ... but at the time, pregnancy tests showed Kathryn was NOT pregnant.

But after filming ended, we're told Kathryn and Ravenel began shacking up in Palm Beach, FL -- where they now live as a couple -- and Kathryn got pregnant for real.