"We got the call at 1 [p.m.] from our doctor telling us it was a girl, and we both wept," Bass tells ET. "We were shocked and elated! I didn't think I could produce a girl! It feels like a miracle baby."
Bass has three sons -- Nathan, Liam and Ensley -- from a previous relationship. The newlyweds announced they were expecting their first child together earlier this month.
Upon finding out that they were going to have a baby girl, the couple turned a party they had planned at their house into a gender reveal party. The celebration was originally a "wedding reception" after they had their televised ceremony on the new season of BIP.
Waddell also documented the experience on her Instagram Stories, saying in a clip, "So, you guys, we just talked to the doctor. We're about to do a gender reveal!"
"It's a girl," shouts Bass in excitement.
"And we're surprising the kids later and we're having them bang open this piñata," added the mother-to-be.
She also posted a sweet pic on Instagram with Bass' sons. "This is our professional Gender Reveal photo! ? IT'S A GIRL!!!! I'm not the only girl anymore! Woo Hoo!!!! ????."
"It was a great surprise, it's a great little miracle," Waddell shared. "We took a pregnancy test in Mexico, and we saw the thing [in Spanish] and were like, 'What does that even say?' So we googled it!"
Hear more of how they found out they were expecting in the video below.