Olivia Wilde has said people go to "great lengths" not to mention her baby bump.
The actress said no-one wants to mention that she's pregnant, in case they've got it wrong.
"That's the worst thing in the world, when you congratulate a non-pregnant woman on her pregnancy," Olivia told Jay Leno on The Tonight Show.
"I find that people will go to such great lengths to not mention it.
"Even when you are clearly - I feel like I'm clearly pregnant - people will be like, 'Hey, would you like a glass of whiskey? Would you like to get on this roller coaster?'
"And I go, 'Oh, well, I'm actually pregnant.' And they go, 'What?!' Unless someone's watching you give birth, I don't think they want to make the wrong call."
Olivia wouldn't reveal the sex of her baby with fiance Jason Sudeikis.
"It's human," the TRON: Legacy actress joked. "We feel good about that."
Olivia and Jason have been dating since 2011 and got engaged in January this year.