The couple, who keep their relationship out of the limelight, are "over the moon" about becoming parents, according to People magazine.
"They literally could not be more excited," a source told the publication. "She's feeling great."
Ryan Gosling and Eva Mendes have been together since 2011
Eva was photographed on Wednesday, although her bump wasn't visible due to the large number of bags she was carrying.
The Hitch actress is thought to be seven months pregnant meaning that the couple, who have been dating since 2011 after meeting on the set of The Place Beyond The Pines, will become first-time parents at the end of the summer.
Ryan, 33, and Eva have always been very discreet about their relationship and are yet to be photographed together this year.
It was only last March that Eva spoke for the first time about her other half, calling him "the best".
Ryan and Eva "could not be more excited" about becoming parents
Eva made the comments about her other half while attending an event in Los Angeles. A reporter for E! News asked her about Ryan's directorial debut, How to catch a Monster, in which she appears.
She replied: "It's basically a dark fairy tale, and I play a woman named Cat. It's a beautiful, beautiful film."
"(Ryan) really created the most beautiful, collaborative, creative atmosphere on set," she enthused, adding: "I know I'm biased. He's the best, he's the best."