Credit: Jason LaVeris/FilmMagic

Kim Kardashian got an early Christmas present: A nephew!

"I'm an auntie!!!!" she Twittered hours after her sister, Kourtney, gave birth to a healthy baby boy in Los Angeles Monday. "My little Mason Dash Disick is here!!! We are so happy!"

Mason weighed in at 7 lbs., 6 oz., and measured 19.5 inches. This is the first child for Kourtney, 30, and her on-again boyfriend Scott Disick.

The baby's arrival didn't stop Kim or mom Kris Jenner from working.

The two still managed to put in an appearance on E!'s Chelsea Lately show.

"We're really, really excited," Kris said, adding that she and Kim were in the same outfits they wore to the hospital.

Host Chelsea Handler then joked, "When I found out she went into labor and had the baby, I said, 'Oh, do we have to get a replacement guest?' And [my producers] were like, 'No, they would never miss a television appearance.'"

"Never, never!" Kim agreed.

Kris then joked, "[Mason]'s in my dressing room — is that wrong?"