The couple announced the news during an Instagram Live video on Friday, September 4, revealing the baby news came as a surprise.
"This is not a drill. This is not a throwback," Brittany said.
"This is real life... We're pregnant again...," Lutz added. "Thank you all for praying for us, supporting us. We're so excited."
His wife revealed she was not expecting to fall pregnant again so soon after losing their daughter "at the beginning of the year".
"That was tough, and still is," she explained. "There still are hard moments. We definitely went through a lot and I still would like to share my journey in some way that I can in the future."
"Here we are with another little promise!"
Kellan recently revealed he and his wife turned to their faith to help them cope after losing their unborn daughter six months into Brittany's pregnancy.
"There is no easy way, but I'm just so grateful for our faith and sharing the blood of Jesus Christ," he told LaPalme Magazine. "We don't have all the understanding in the world. There are some tough days where you'll see the sonogram, and you'll see her cute little nose that she had. But then again, we are so grateful she is in heaven, and we remind ourselves that she didn't have to come into this world and deal with heartbreak, and she gets to be with Jesus. It's bittersweet."
The actor also revealed pair would be trying for another baby in the future, smiling: "I'm happy to trust that one day we'll have a big family."