A rep for the Lord of the Rings actress confirmed the pregnancy to People following weeks of speculation.
Liv and Dave have kept their romance very low key and far away from the spotlight, but it's believed they met via mutual friend Kate Moss earlier this year.
A source told The Sun earlier this year: "This is the real deal. Dave and Liv are madly in love. They mix in the same circles because Dave hangs out a lot with a group that includes Kate and Stella McCartney."
This will be Liv's second child.
The 37-year-old daughter of Aerosmith frontman Steven Tyler is already mum to son Milo, her child with ex-husband Royston Langdon.
British born Dave, a football agent, is also a dad, sharing son Grey with ex-wife Davinia Taylor.
Stars who welcomed babies in 2014:
Copyright: Rex Features WENN/Matthew Taylor/Daniel Deme/Josiah True/Matthre Taylor/Joe
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