Catelynn and Tyler are gearing up for the arrival of their "rainbow baby" -- and in a special sneak peek of the upcoming Teen Mom OG season, the couple is readying Nova for her upcoming role as big sister.

"You know why we're making pink and blue cupcakes? Why are we having a party tomorrow? Mommy has a baby in her belly!" Cate tells her visibly excited three-year-old daughter in the video above, adding that they will soon find out if it's a boy or a girl.

"Yeah!" Nova enthusiastically replies, with a huge smile.

But does Nova want a little brother or a little sister? And how do her loving parents react to her answer? Watch the heartwarming clip, then be sure to watch Cate's pregnancy journey on Teen Mom OG, premiering on Monday, October 1 at 9/8c -- and stay with MTV News for Baltierra baby updates!