Lindsay Lohan -- She's Gonna Be a Big Sister!

Exclusive

There's going to be a new addition to the Lohan litter -- but fear not ... it's not Lindsay who's preggers. TMZ has learned Lindsay's gonna have another sibling, courtesy of Kate Major and Michael Lohan.According to our sources, Kate is in her first trimester. But get this ... the restraining order she had against MiLo was lifted lifted only a few weeks ago ... so the act of conception was illegal.

Our sources say Michael has not told Lindsay or any of his other children the happy news yet. So Lindsay, we're pleased to inform you you're gonna have a brother or sister to whom you can teach the ropes.

A rep for the couple confirms the pregnancy, but wouldn't comment further.