"Last night at 10:28 PM Ada James Akins was born and it was truly the most amazing thing I have ever experienced," Rhett, 27, wrote. "I can't believe that we have two daughters!!"
The "Crash and Burn" singer added that he has a "newfound respect for moms" following his wife's long labor of 36 hours.
"My wife @laur_akins labored for almost 36 hours," he continued. "She is by far the strongest human being I have ever met and I have a newfound respect for moms around the world. We are so excited that Willa Gray has a baby sister! Thank you Lord and everyone for the prayers."
Rhett added the "#outnumbered" hashtag, referencing the couple's recently adopted daughter, 1-year-old Willa Gray.
Akins also posted the same snap to her Instagram account, referencing the 36-hour labor, "Only Jesus got me through that, and she is sooo worth it. She weighs 7 pounds 3 ounces and is so stinking cute I just can't take it!"
The proud mom added that the new family of four are "doing well now, just resting."
"Thank y'all for the prayers -- she is a miracle and God is so good," she continued. "Thomas Rhett and I are so [in] love with her. And Willa Gray is so excited to finally have a baby sister."
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Model Irina Shayk and hunky Bradley Cooper had their first child in April 2017 -- and with those genes, she's sure to be a knockout!
Jimmy Kimmel's wife, Molly, gave birth to their second child on April 21 -- and just hours after his delivery, he was slated for open heart surgery. Kimmel opened up about the terrifying events, getting emotional on-air as he detailed his newborn's difficult journey. Thankfully, little Billy is now on track to a full recovery.
Ciara and husband Russell Wilson welcomed baby Sienna Princess on April 28 after months of public anticipation! Ciara documented the pregnancy heavily via social media up until the big day.
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He largely stays out of the spotlight, but reports confirmed that Jesse Eisenberg and girlfriend Anna Strout welcomed their first child in early 2017.
Benedict Cumberbatch and his wife, Sophie Hunter, grew their family by one with the birth of son Hal Auden on March 3. His big brother is Christopher, born in 2015.
Natalie Portman and her husband, choreographer Benjamin Millepied, had their second child on Feb. 22. Her name is Amalia Millipied.
Zooey Deschanel and her husband, Jacob Pechenik, had their second child in spring 2017, her rep confirmed. Little Charlie's older sister is Elsie, who was born in 2015.
Tori Spelling and Dean McDermott increased their already-full household with their fifty child, born on March 2. Little Beau is the perfect addition to "our big, beautiful and healthy family," Tori told People.
Gal Gadot gave birth to a Wonder Woman of her own in March 2017! She and her husband, Yaron Versano, were already parents to daughter Alma, born in 2011.
Backstreet Boys star AJ McLean and his wife Rochelle Deanna Karidis had the perfect name for a sure-to-be musically inclined baby: Their daughter, Lyric, was born in March.
Chad Michael Murray and Sarah Roemer excitedly announced the birth of their second child, a girl, in March 2017. Their first son was born in 2015.
"Grey's Anatomy" star Camilla Luddington and her boyfriend Matthew Alan announced the birth of their baby girl, Hayden, in spring 2017.
Pharrell Williams and Helen Lasichanh welcomed triplets (!!!) in January 2017. The little trio have an 8-year-old brother, Rocket. (By the way, this is the couple just a few months after Helen gave birth!)
Armie Hammer's wife, Elizabeth Chambers, gave birth to their second child in January 2017. The baby boy, Ford Douglas, has a big sister in 2-year-old Harper Grace.
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In February, the couple announced that they were not only expecting, but they were also in the process of adopting a baby girl from Africa. The GRAMMY nominee later told ET's Sophie Schillaci that he planned to make any and all adjustments necessary to be able to take his family on tour.
"We've already added a crib to our tour bus," he said. "We're trying to make all the right alterations to make sure that our kids can come [on tour]."
The singer added that he made sure to stock up on sleep in anticipation of the couple's new arrival. (Smart move, Rhett!)
"I wish I got a break, but I doubt it's gonna happen," he told Schillaci. "We're just gonna try to integrate them into our lives as much as possible and try to just create as much of a normal lifestyle as possible. I'm getting all the sleep I can right now, because I know that I'm about to not sleep ever."
As for having two girls? Rhett is elated!
"Before we even started wanting to have kids, I wanted girls. I'm just gonna love them so much," he confessed. "I'm nervous about it as well -- I don't want to deal with boyfriends down the road, you know?"