The `Fool`s Gold` actress - who has a seven-year-old son Ryder with ex-husband Chris Robinson - and boyfriend Matt Bellamy are said to be expecting their first child together, with the 31-year-old blonde beauty 14 weeks into her pregnancy.
A source told Us Weekly magazine: "It was not planned, but they are excited and embracing it.
"She is happy for Ryder to finally have a brother or sister."
Kate and Muse rocker Matt, 32, have been dating for around nine months.
The couple recently spent Christmas with both sets of parents, with the British singer flying his mother Marilyn and father George - who are divorced - out from their homes in Exeter, South West England to California to enjoy a big family celebration with Goldie Hawn and Kurt Russell.
A source said before the meeting: "It will be the first meeting of both sets of parents. Things are getting serious between Kate and Matt and it`s the obvious next step.
"Christmas is the only time they can get everyone together and the festive season is also the best time to do it.
"Matt`s parents divorced when he was a teenager but they maintained a relationship."