Kourtney Kardashian

Kourtney Kardashian's unplanned pregnancy was the result of bad memory: The eldest Kardashian says she got pregnant after forgetting to take her birth control pill.

"This probably sounds so dumb, but there's so many times I'll forget to take my pill and I don't think it's that big of a deal," the 30-year-old, who's due around Christmastime, told Ryan Seacrest's KIIS FM radio show. "It's just so stupid."

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Kardashian, who confirmed that the baby's father is her on-again boyfriend Scott Disick, thought she might be pregnant when she got "really nauseous" and her breasts became "huge and they were so sore."

So taken aback by the unexpected news, Kardashian said she charged her sister, Khloe, with telling their family. Her stepfather, Bruce Jenner, had the "most realistic" reaction. "[He said], 'You guys are acting like you're getting a little puppy dog! You need to take this seriously.' ... He freaks me out the most," she said.

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Younger sister, Kim, was surprised as well because she always thought she'd be the first to have children, Kardashian said. Kim, who recently split with Reggie Bush, even had baby names picked out.

"I don't think she was envious, but who knows. Kim gets mad if I get, like, the car that she wants. She, like, freaks out," Kardashian said. "Out of all of us, I was definitely the last one that was even thinking about it."