"They are excited and happy even though it was unexpected," an insider told US Weekly.
David, 42, already has a nine-year-old daughter with ex-wife Courteney Cox.
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He has been dating former Entertainment Tonight reporter Christina on and off since 2011, and earlier this year described her as "an amazing person, a beautiful person."
Last month, Christina denied reports she was engaged to the Scream 4 actor.
"We are totally happy and together in love, but we are not engaged," she told US Weekly. "It's just a rumour that got out of control."
David and Courteney have remained on very good terms since their split in October 2010.
In an interview on The Ellen DeGeneres Show in January, Courteney described David as "my best friend in the world".
"I don't recommend divorce in general," she said. "But he is my best friend and we've both grown and changed, and I think we both appreciate each other more."