Reps for the actors did not immediately respond to request for comment.
This will be the second child for the pair, who already have a 19-month-old daughter, Esmeralda Amada.
The private duo began dating in 2011 after meeting on the set of The Place Beyond the Pines, in which they played parents.
Gosling, 35, and Mendes, 42, managed to keep their first pregnancy a secret for several months, with an insider explaining that Mendes “just hasn’t ever wanted anyone beyond her close friends and family knowing more than they had to. And Ryan is certainly private and always has been.”
While the actress had said in the past she wasn’t planning to get married or have children, “with Ryan things are different,” a friend told People. “She’s very independent, but she’s content with her partner. She finally found the person she really wants to be with.”
Gosling recently opened up about his happy family life in an interview with Hello! magazine, saying, “I know that I’m with the person I’m supposed to be with.”
The stars have kept their daughter out of the spotlight since her birth, but a source previously told People that Gosling is “madly in love” with Esmeralda, and Mendes is loving her life as a new mom.
“She’s never been happier,” the source said of Mendes. “She and Ryan are just in awe of this beautiful person they created. It’s a really special time for them.”