"She's very independent but she's content with her partner. She finally found the person she really wants to be with," the magazine quotes Mendes' friend about the Hitch star's relationship with Gosling.
This will be the first child for both Mendes, 40, and Gosling, 33. The couple has been dating since 2011, after meeting on the set of their 2012 film The Place Beyond the Pines.
Earlier this year, Mendes flat-out denied rumors that she was pregnant when she appeared on The Ellen DeGeneres Show, which started when she refused to go through scanners at the airport.
"It's so ridiculous," she told DeGeneres. "It all started because I didn't want to go through the scanners at the airport. You know those X-ray scanners, which are really creepy? They basically see you naked, right? And not only that, but there's a radiation aspect to it, so I always opt out."