Kim Kardashian's oldest sister also gave her fans a little glimpse of her baby boy by sharing a picture which showed his tiny hand holding on to Kourtney's.
Kourtney and Scott Disick welcomed Reign Aston on 14 December, which coincidentally is also their first born son Mason's birthday.
"The family is freaking out with joy," a source told E! News at the time. "What an amazing coincidence."
The reality TV star revealed she was expecting a son on a recent episode of Kourtney & Khloé Take The Hamptons.
"I am so happy that we're having a boy," Kourtney, 35, said. "It's going to be perfect for our family."
The raven-haired beauty, who is also mum to five-year-old Mason and two-year-old Penelope, added that her eldest child played a starring role in the big reveal.
"Mason ran in here with the blue stick," Kourtney was seen telling her younger sister Khloé Kardashian. "There was a red star or a blue star."
"I wanted a girl," admitted Mason, until Khloé – or Auntie KoKo as she is affectionately called – helped sway his attitude.
"Girls don't like aliens and Lego guys," Khloé told the adorable youngster. "When you have a brother you can say 'Look at all my Lego guys, brother.' You get to be rough and you can boss him around because you're the older one."