The couple, who've been engaged since 2013, already have a son, Otis, together.
It was actress Olivia who shared the news, posting an adorable black and white photo to her Instagram account which shows her relaxing with Otis - both holding their stomachs with her gorgeous bump on display.
The caption simply reads: 'Matching baby bumps.'
Little Otis, who turns two on Wednesday, looks excited at the prospect of becoming a big brother. Olivia and Jason have been open about how much they love parenthood since welcoming their first son into the world last April, and it seems they're no strangers to the sheer pride many other parents will identify with!
Jason recently told reporters he wanted to expand his family, joking that producing a perfect child in Otis had convinced them to have more. When asked about Olivia's revelation that 'everything' makes her want to give Otis a sibling, he told E!: 'Not everything. My own kid does, because I made it with Olivia and that seems like a good cocktail.'
'I'm all for it', he added.
Olivia can often be found sharing hilarious, 'real parenting' pictures to her Instagram account, as these prove!
Speaking to E! News about what motherhood means to her, she said:
'The gift of motherhood is the selflessness that it introduces you to, and I think that's really freeing,' she said. 'And in terms of being a storyteller, I think it allows you to put yourself in other people's shoes... the empathy that it slugs you with, being a mother. And I think it makes you a better storyteller.'
Olivia, 32, and her fiancé Jason, 40, are currently in Hawaii to promote Jason's new film Angry Birds, for which he voices one of the characters.
The pair first began dating in November 2011 and moved in together in 2012. Olivia, who was previously married to Italian Prince Tao Ruspoli for nearly eight years, has confessed that she fell 'blissfully, hopelessly, wildly in love' with Jason.