The two met on the set of the long-running series, That 70's Show, and officially started dating in 2012. They welcomed daughter Wyatt Isabelle in October 2014.
A source close to the couple told People, "Ashton genuinely lights up when talking about Wyatt and the little one on the way. Everything about being a father is natural to him."
Kutcher, 38, and Kunis 32, married last July and the actress told Entertainment Tonight in February that she was ready to expand her family.
"I'm not pregnant," she said earlier this year, "But absolutely, I do want to have more kids one day, for sure."
In an interview with Ellen, shortly after the birth of Wyatt, Kutcher gushed about his wife.
"She's the greatest mom," he said. "I go to work every day, and I come home and she's perfect. And it just seems like everything went amazing. And I know that something probably didn't go amazing, but she never tells ... it's unbelievable. She's incredible."
Kutcher currently stars in the Netflix series, The Ranch, and Kunis stars with Kristen Bell in the upcoming comedy Bad Moms, which hits theaters July 29.