Catelynn Lowell and Tyler Balterria's family is growing -- and Nova is going to be a big sister!

“The big news is that we are expecting our rainbow baby," Lowell divulged to Us Weekly. "This baby is our rainbow after the storm. I’m super excited." A brand-new Baltierra bundle of joy!

Cate continued: "It was a huge shock at first. We were not planning it, especially after the miscarriage and how I went downhill with my mental illness. We were going to wait a really long time. We were using protection and everything and still got pregnant. This baby just wanted to be here. It was very unexpected.”

Cate's proud hubby shared a snapshot of his wife -- and gushed that he was "so excited to finally reveal our big news."

Offer your congratulatory messages to the MTV clan in the comments -- and be sure to watch the brand-new upcoming season of Teen Mom OG premiering on October 1 at 9/8c (a glimpse of what's to come can be seen below).