Jacob Bryan Fisher was born early Monday morning, she said in the social media post. Underwood included photos of the newborn with members of the family, including 3-year-old Isaiah Fisher.
"Jacob Bryan Fisher entered the world in the wee hours of the morning on Monday...his mom, dad and big brother couldn't be happier for God to trust them with taking care of this little miracle! Our hearts are full, our eyes are tired and our lives are forever changed. Life is good," she captioned the photo gallery.
Underwood announced her pregnancy in August, saying she was "over the moon."
"This has just been a dream come true with the album and baby news and all that stuff. We're just so excited and just so glad you guys can share this with us and be a part of this," she said at the time. "Love you guys! We will see you on the road in 2019!"
In September, Underwood revealed she had three miscarriages between 2017 and 2018.
"It happens. And that was the thing, in the beginning it was like: 'OK, God, we know this is, just wasn't Your timing. And that is all right. We will bounce back and figure our way through it.' And got pregnant again in the spring, and it didn't work out," she said. "Got pregnant again, early 2018. Didn't work out. So, at that point, it was just kind of like, 'OK, like, what's the deal? What is all of this?" Underwood said during an appearance on CBS Sunday Morning.