During an interview with Ryan Seacrest‘s KIIS-FM radio show Thursday morning, Kourtney Kardashian singled out a mix-up with birth control as the explanation for her baby-on-the-way, due this Christmas. “This probably sounds so dumb,” she admits, “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. I’ve done that several times and never really thought about it. It’s just so stupid.”

Now five months along into her first pregnancy, Kourtney says that her unexpected baby news took little sister Kim Kardashian by surprise as well. “She definitely thought it was going to be her first,” Kourtney muses. “Kim has, like, every baby name picked out.” And while Kourtney doesn’t think Kim is “envious” of the pregnancy per se, siblings will be siblings! “Kim gets mad if I get, like, the car that she wants,” Kourtney noted. “She freaks out.”

It was Kourtney’s other little sister Khloe Kardashian — with whom she co-stars in the new E!reality show Kourtney and Khloe Take Miami — charged with the task of sharing the bombshell with the rest of the family. According to Kourtney, step dad Bruce Jenner‘s reaction was the most sobering of all. She recalls,

Over the last few months Kourtney kept her bump under wraps by wearing unbuttoned jeans and a big shirt, but with the pregnancy finally out in the open she can focus all her energy on preparing for what lies ahead. First and foremost is choosing a name! No matter what the sex, baby-on-the-way’s name will begin with a letter other than K, Kourtney promises.

There is also the issue of which last name baby will have — mom’s, or that of Kourtney’s recently reconciled boyfriend Scott Disick. “I don’t know [if we’re getting married],” Kourtney admits before adding,

— Missy