"Tiny hands and tiny feet...God has blessed us with an amazing gift! Isaiah Michael Fisher - born on February 27. Welcome to the world, sweet angel!" she wrote.
Underwood, 31, announced her pregnancy on Labor Day by posting a sweet picture with her dogs.
"In honor of "Labor" Day...Ace & Penny would like to make an announcement. Their parents couldn't be happier!" the country music star wrote.
The picture featured her two dogs wearing shirts that said they were going to be a big brother and big sister to a new baby.
A couple months later, Underwood's co-host at the Country Music Awards, Brad Paisley, let slip that she was having a baby boy.
Underwood later told ABC Radio that she had planned to announce the baby's sex during the show all along, but she didn't let Paisley in on the secret until the very last minute.
"Brad's got loose lips. He's a blabbermouth!" Underwood said with a laugh. "So, I was like, 'I'm not gonna tell you until we're ready for the rest of the world to know, 'cause I know you'll tell people.'"
She and her husband both had a feeling they were having a baby boy but had the doctor write down the baby's gender for them to look at when they were ready. They took the envelope containing the news of the baby's sex with them to a nice restaurant.
"[It was] just the two of us in a nicer setting, but we both knew," she said. "So, that was just confirming our suspicions. We didn't really speak of it before, 'cause we were both just like, 'Whatever. It's all good.' But we knew it was a boy."