The former House and The OC star continued her promotional duties for new movie Her with an appearance on The Tonight Show With Jay Leno.
Talk inevitably turned to her pregnancy, prompting Olivia to say: "I find people will go to such great lengths to not mention it. I feel like I am clearly pregnant but people will be like, 'Hey, do you want a glass of whiskey? Go on the rollercoaster?' I say, 'I'm actually pregnant' and they go, 'WHAT?'
"I feel unless somebody is watching you give birth they don't want to make the wrong call."
As for whether Olivia and fiance Jason Sudeikis are expecting a girl or boy, she remained tight-lipped, joking: "It's human so we feel good about that."
Olivia wore a glamorous figure-hugging black dress for her appearance on the show but later changed into comfy jeans and a loose T-shirt for her ride home... but not before swiping a souvenir from the studio.
"I broke into @jayleno's dressing room and stole one of his jackets for my fetus," she tweeted.
Olivia and Jason announced at the end of October that they are expecting their first child together, but have kept quiet on any additional details.
They became engaged earlier this year.
Olivia's not alone! More pregnant stars in Reveal's gallery below:
Copyright: Vimeo / Weiss Eubanks