A representative for the actress confirmed the good news to People magazine.
US Weekly magazine also revealed the news and quoted a source as saying: "They both want kids, it's one of the reasons they got a bigger place, in a neighbourhood that you can raise a family. They couldn't be more excited.
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Fatherhood has been on John's mind for a while. The 33-year-old star, who is best known for his part in the American version of The Office, got some practice in while writing Promised Land with Matt Damon.
"We actually wrote with children climbing all over him, and he was doing bath time and dinner and everything, and we still got it done," he said.
Matt was one of guests along with George Clooney, Stanley Tucci, David Schwimmer and Meryl Streep.
Sweetly John couldn't believe it when the British beauty agreed to go out with him.
"It was one of those things where I wasn't really looking for a relationship and I was thinking I'm going to take my time in LA," he told Ellen DeGeneres. "Then I met her and I was so nervous. I was like, 'Oh god, I think I'm going to fall in love with her.' As I shook her hand I went, 'I like you.'"