This is the first child for the couple, who began dating in 2011 as they costarred in the film The Place Beyond the Pines.
And while there were rumors that the two's relationship had become a rocky one, the new parents, who kept Mendes's pregnancy under wraps for months, couldn't be more thrilled about starting a family together.
"Their lives completely changed as soon as Eva got pregnant. They used to travel all over the place together and go on hikes with their dogs. [But in the past few months] she barely left the house," a source close to the couple tells People. "Ryan has been taking care of her, picking up food and running errands. He has been very supportive of keeping things a secret."
"This has been a long time coming," the confidant says of the hush-hush couple. "Ryan and Eva definitely discussed children before. She knows how much he wants to be a dad and he's going to be an amazing father."
Their new arrival begs one very important question: Will Gosling put a ring on it anytime soon? Celebrities are loving a joint engagement and pregnancy, so they wouldn't be the first to blend the two memorable moments together. Besides that, we're sure Mendes will make a gorgeous bride, and even more importantly, Gosling a dashing groom.
We sincerely hope upon his daughter's arrival, Gosling say "Hey, girl."