The 41-year-old actress – who plays Ellie Miller in the ITV detective drama – told the Mirror she and her novelist husband Ed Sinclair will become parents to their third child later this year.
Olivia said: "We're all very excited."
The couple already have sons Finn, nine and seven-year-old Hall together.
Olivia revealed the news as she spoke about why she can't take part in a big-screen remake of her short film The Karman Line, which Spike Lee is directing.
Olivia told the paper: "I love Spike Lee and would certainly take his call but he'll have to wait a while before we start anything."
Olivia, who's also known for her role as Sophie Chapman in Peep Show and vicar's wife Alex in Rev, recently revealed her fears about her acting future.
She told the Sunday People: "It's harder to get roles as you get older.
"I have a lot of friends that should be working and aren't and that's a real mystery and needs to change."
And she said she felt that it isn't her acting ability that gets her parts.
Olivia has also revealed she's careful to make sure her husband knows he is appreciated.
She said: "When you've got children it's easy to do that thing of keeping a tally of who woke up earliest and whose turn it is to put them to bed.
"But I think the important thing is to appreciate and love each other and to show that appreciation."