"I was about two months pregnant. I was really sick," the star of This Is 40 said when she returned for the Dec. 17 episode of the NBC talk show. "I was very nauseous. I had really bad morning sickness."
Fox continued, "It was so bad for me. . . I was convinced that I was, like, maybe birthing a vampire baby like the one in Twilight -- you know what happens to Bella, where she's in cold sweats all the time -- and I felt like that was happening because I had no vitamins and nutrients and I was just nauseous."
Luckily, Brian Austin Green's wife of two-and-a-half years didn't suffer for long. "The first trimester was really rough and then I got to about 12 weeks and then it was fine," the actress said. "It was really a perfect pregnancy after that."
Fox -- already stepmom to Green's son Kassius, 10, from his previous relationship with actress Vanessa Marcil -- also opened up about giving birth Sept. 27 at L.A.'s Cedars-Sinai Medical Center. (Coincidentally, Reese Witherspoon and Jim Toth welcomed a son, Tennessee, at the same hospital that same day.)
"I thought I was going to be tough and the nurses would be like, 'She's a warrior princess! She doesn't need an epidural! She's amazing! We're such big fans!'" Fox admitted. "As soon as I got out of the car I was already crying for an epidural. I was asking the security guard for an epidural!"
This Is 40 is in theaters Friday, Dec. 21. The Tonight Show With Jay Leno airs weeknights at 11:35 p.m. on NBC.