Leo is the couple's first biological child, but they are no strangers to parenting: Reese and Chaplow adopted Chaplow's niece and nephew in 2011.
Both Leo and Reese were in good health after the birth, the couple wrote on Facebook. Reese and Chaplow told CNN Leo "eats like a champ, sleeps well, and gives us lots of cuddles."
"We are so lucky to have been able to welcome him into our lives and cannot wait to see who he grows up to be," they said.
Reese and Chaplow have documented their parenting adventures and posted updates throughout Reese's pregnancy on their Facebook page
. They made a video
announcing the pregnancy in January.
Reese, who was assigned the female gender at birth, posted a video
in March explaining his decision to carry a baby as a transgender man.
"I'm OK with my body being a trans body," he said. "I'm OK being a man who has a uterus and has the capacity and capability of carrying a baby."
Reese told CNN in June
that he had stopped taking testosterone to prepare for the pregnancy.
"We've been under medical supervision the entire time," he said, "to make it as healthy and safe as possible."
In addition to the "normal pregnancy stuff" like cravings -- Reese's was French fries from McDonald's -- the couple has had to face criticism and negativity online.
"We find that behind the shroud of anonymity, people feel pretty empowered to tell us what should happen to us, to our children, to our family," Reese said. "The reason why you have a kid is because you want to see more love in the world, and remembering how difficult that's going to be, it's hard."
But since Leo was born, the couple has received an outpouring of support on Facebook and media attention from around the world.