Anger Management star Brian, 42, confirmed the news this weekend after months of secrecy.
It will be 29-year-old Megan's third child with Brian, even though she filed for divorce in August 2015.
Earlier this week, the Transformers actress attended the CinemaCon Paramount Gala opening night event in Las Vegas' Caesar's Palace.
Audiences were surprised when Megan was seen sporting a prominent baby bump.
Days later, Megan and Brian were seen together at Long Beach's 40th and final Toyota Grand Prix/Celebrity Race.
Confirming they are expecting, he told People magazine at the event: "At my age, to be having three babies, is crazy. I'll be 43 this year.
"You know, nothing is planned. None of them are ever planned. You kind of just go with it."
The Hollywood couple - who met on the 2004 set of Hope & Faith - also have two sons, Noah, three, and Bodhi, two.
They split last year after five years of marriage and 11 years on/off as a couple.
Brian also has a 14-year-old son Kassius with former Beverly Hills, 90210 co-star Vanessa Marcil.
A source told People following the pregnancy news: "Of course Brian is the dad! They never stopped loving each other and they will never stop being devoted parents to their boys."
It's thought the divorce is now on hold, and they are believed to be sharing a rented home together in Bel Air with their two sons.
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