Jennifer Garner (search) may have let the cat out of the bag.

The "Alias" star appeared on the "Tonight Show" Tuesday night and refused to answer Jay Leno (search) when he asked if she was having a boy or a girl. But, later in the show, Garner talked about her expanding belly.

"You can just start to feel really pregnant, like you are the hugest person on the face of the planet," she said. "I felt bigger and bigger, like she _".

The audience cheered and Garner laughed as she stopped in mid-sentence.

A sense of humor has served the actress well, especially since her pregnancy has been written into the show. Her character, Sydney Bristow, discovers that she's pregnant with fellow agent Michael Vaughn's baby.

"My stand-in has a bump, and my stunt double has a bump," she revealed. "The other day, I walked in, and my stunt double looked a little different to me." It turned out they increased the size of her bump.

Garner married actor Ben Affleck (search) in June. The baby is the first child for both.