Model Alexis Knapp only dated Ryan Phillippe for a few months, but the stunning brunette is claiming she's carrying his baby. The two split in September 2010.

The mommy-to-be made the big announcement at a Hollywood party last month. "Alexis hadn't even talked to Ryan before that party," a sources tells Us.

The 36-year-old Lincoln Lawyer actor is already father to daughter Ava, 11 and son Deacon, 7 ... whom he had with ex Reese Withersppon.

As for how he's handling the news, the source adds:

    "If it's his, he'll take responsibility. Reese has been really supportive. She knows that Ryan is a good dad and she respects that."

Phillippe is currently dating Amanda Seyfried. Witherspoon is engaged to fiance Jim Toth. Find out when they're tying the knot ... after the jump.