"Beer Belly, Bella Belly, BABY BELLY!!!" Carly, 33, captioned a photo of the couple and Bella, making the announcement.
"We so SO EXCITED to FINALLY announce Bella's little brother or sister will be here in November!!! (If you wonder why I haven't been on social media as much... well, this is why!)"
In the photo, Evan, Carly and their 14-month-old daughter Isabella "Bella" Evelyn Bass were all showing off their stomachs.
Evan was holding a six-pack of beer, Bella had a sippy-cup in hand, and Carly was showing off a series of images from her sonogram.
Evan was smiling, while Carly appeared overjoyed. And Bella had an adorable surprised look on her face.
Evan, an erectile dysfunction expert who also has three sons from a previous relationship -- Nathan, Liam and Ensley -- shared an Instagram post of his own.
"Ok, so I think I finally figured out what's causing all these babies. Baby number 473ish coming soon! (Im currently carefully curating this babies Instagram page to be released with ointment line in 2054.)" Evan joked.
They welcomed Bella into the world in February 2018 and told Us Weekly in March of that year they were going to begin trying to conceive for Baby No. 2 shortly.
"We're planning on trying to have another little one and getting pregnant this year," Carly told Us at the time. "I always tell [friend and fellow Bachelor in Paradise alum Jade Roper], 'You go first, and I'll go second.'"
Jade -- who co-hosts the "Mommies Tell All" podcast with Carly -- and her husband Tanner Tolbert announced in January 2019 they are expecting their second child together.
Jade and Tanner welcomed daughter Emerson, now 20 months, in 2017.