Kourtney Kardashian's Family Welcomes Baby Boy Mason Dash Disick

'I'm an auntie!!!!' younger sister Kim posts on Twitter.

Kourtney Kardashian gave birth to her son, Mason Dash Disick, on Monday, and obviously the Kardashians want to share their excitement with the world.

Kourtney's younger sister took to her Twitter account, posting, "I'm an auntie!!!! My little Mason Dash Disick is here!!! We are so happy!"

Keeping Up With Kourtney And Scott

Kim, along with mom Kris, stopped by E!'s "Chelsea Lately," where they said they couldn't cancel their appearance on the show, even sharing they were wearing the same outfits on the show they had worn to the hospital. "We have to think of another name [besides grandma]. This is getting crazy. We're really, really excited and it's been an amazing evening," Kris said. "[Mason]'s in my dressing room — is that wrong?"

Rob Kardashian's ex-girlfriend Adrienne Bailon also turned to Twitter to wish Kourtney and Scott all the best in their new roles as parents. "Congratulations!!! @KourtneyKardash I LOVE YOU!!!," she wrote, adding, "And #Masondashdisick ha! Happy Birthday!!!"

Kourtney, who announced her pregnancy in August as she was promoting her show, "Kourtney & Khloé Take Miami," spoke to E! News about adding son Mason to the family, noting that she was excited to be having a boy. "A boy is definitely needed in this family!" Kourtney said in October. "Scott and I are thrilled to add a little boy to the bunch."

She later shared that she thought her younger sister Kim would be the first to start a family. "She definitely thought it was going to be her first. Kim has, like, every baby name picked out," the "Keeping Up With the Kardashians" star said. "Out of all of us, I was definitely the last one that was even thinking about it."

Between Kourtney's new baby and Khloé's wedding, it's been a big year for the Kardashians, who were chosen as MTV News' #9 Women of the Year.