Kim spilled the beans while sharing a funny story about her four-year-old daughter North West.
"My daughter thinks she's really tricky. We had a baby shower over the weekend and I thought, 'I really do want to have a baby shower, because I want her to feel like something's coming and for her to really understand it,'" Kim explained, referencing her tea-party themed baby shower complete with a cherry-blossom forest on Saturday.
"So people brought toys and gifts, and [North] was opening them all up the next day, and she said, 'Mom, since Baby Sister isn't here, I think I need all of her toys in my room, and I'll play with them just to make sure they're all okay for Baby Sister.' And I'm like, 'Okay.'"
Kim didn't seem to realize that by sharing North's pet name for her sibling on the way, she leaked her third child's gender.
"It is, yeah," Kim noted. "North is very excited about that... She's so excited. Let's see if it lasts until when I'll have to explain to her, 'Okay, four in the morning, when you come in my room, I have to be with Baby Sister. I have to feed her.' So we'll see how it goes."
Kim and Kanye are struggling to come up with the perfect name for their little bundle of joy, who is reportedly due in January 2018.
"We don't have a name... [At the shower], I had everyone write a name down on a tile to see if anything sticks. We're freaking out. We don't have a name," the reality TV star said, adding that she didn't fall in love with any of her friends' suggestions at the party.
Ellen then tossed out a few names as options, but she wasn't serious. However, Ellen's idea of the name "Star" seemed to interest Kim.
"You know what's so funny? I'm not going to name her Star, but my daughter picks that name all the time," Kim told Ellen, explaining that North names all of her toys Star West.
"[But] I'm not vibing on it. It's so hard. And I do like different names -- like short, easy to spell, and one syllable. That's my kind of vibe."
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Ellen insisted she had actually come up with the name Saint for Kim and Kanye's second child together, a baby boy, but that the rapper has denied her involvement ever since.
As for whether Saint understands he's going to have a sibling soon, Kim reasoned, "I don't know if my son gets it yet, especially since I don't have the belly. It's a different experience."
She continued, "So I said to my sisters, 'Would I be really creepy if I just had someone dress up like a big stork and drop the baby off at the front door?' And have to explain that? Isn't that what we heard when we were growing up? About storks?"
Ellen, however, firmly advised Kim against doing that so that her children won't grow up confused.