The Family Guy star, 32, and actor partner Ashton Kutcher are expecting their second child, a rep confirmed to People magazine.
They're already parents to daughter Wyatt, one.
Mila and Ashton, who met on the set of sitcom That '70s Show, have always kept their private life away from the cameras and spotlight.
They didn't confirm the first pregnancy until three months after rumours started swirling, with Mila later saying: "We were able to hide it for a very long time, and that's amazing."
Their engagement was also kept private for two months, while they also managed to tie the knot in secret, only letting the public know after the ceremony had taken place.
Chatting on the Ellen DeGeneres Show earlier this year, Ashton, 28, said: "It was a ninja effort. We really didn't want helicopters at our wedding, it was a legitimate concern. I was putting things on social media that we were in different locations. We didn't want to be screaming our vows at each other being like, 'Forever... did you say never?' so we went really under the radar with it."