"Tiny hands and tiny feet...God has blessed us with an amazing gift! Isaiah Michael Fisher - born February 27," the fourth-season American Idol winner captioned a photo on Facebook of her newborn Tuesday.
During the 2014 Country Music Awards in November, co-hosts Underwood and Brad Paisley exchanged a few jokes which resulted in Paisley spilling the beans about whether Underwood's baby was going to be a boy or a girl.
Underwood, 31, first announced her pregnancy in early September of last year by posting a photo on Instagram of Ace and Penny, her two dogs with Fisher, sporting their respective "big brother" and "big sister" shirts.
Fisher, a 34-year-old professional hockey player for the Nashville Predators, hinted at what the baby's name could be on Twitter at the time.
"We haven't picked names yet but it's looking like Fly is gonna fly. #boyorgirl #flyfisher," he jokingly tweeted.
Underwood and Fisher got married in Georgia in July 2010 following their December 2009 engagement. The couple had dated for about a year before Fisher proposed.