In fact, the 29-year-old nominee for Best Actress at this weekend's Oscars poked fun at the 'whole birthing plan concept'.
She said: "Sorry, but what is the plan apart from to get it out? There isn't an option to sort of keep it in, is there?"
The Imitation Game star, who is expecting her first baby with husband James Righton, added: "So, I'm assuming my plan is to get it out. But apparently there's more to the plan than that. I don't know what that is. Still, my plan is to get it out. It will come out."
Ellen pointed out that 'back in the day, people would be in the field and have a child'.
Keira joked: "Yeah, just drop it out and keep working. Pick it up...yeah, that's my plan."
She added that she can't wait for her pregnancy to be over so she can have a drink.
"I can't drink. I'm up for an Oscar and I can't drink at the Oscars," she said.
"I went to the doctor - you have to get lots of doctor check ups when you're pregnant - and she asked if I had any questions. And I said 'Yes! When can I drink!? Please! I just want a margarita'."
But she revealed her husband James hasn't been abstaining.
She said: "My husband's having a great time. I'm eating for two and he's drinking for two.
Keira also joked that she was concerned about the perils of having a baby boy.
"Somebody said to me the other day the problem with boys is - sorry this is horrible - but when you're changing their nappies you can get the wee in your face," she said to knowing laughter from the audience.
"Yeah you see? I've got a lot of people nodding at me, but I never knew that before so it was terrifying."
Keira was later joined by dad-to-be and her co-star Benedict Cumberbatch, who is expecting his first child with director Sophie Hunter, whom he married at the weekend.
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