Mila, 32, and Ashton, 38, are already parents to their 20-month-old daughter Wyatt Isabelle, who they welcomed on 1st October 2014.
According to Harper's Bazaar, the actors confirmed the happy news via a representative to People magazine.
The news comes as no surprise to most, after Mila revealed to Ellen DeGeneres, while appearing on her talk show back in May, that she wants to have more kids.
When Ellen asked 'Do you want to have more children?' the mum-of-one replied, 'Yes, absolutely, yes yes yes.' Ellen then asked 'Are you trying?', to which Mila laughed 'As in am I having sex? [Laughs] Yes!'
Ashton and Mila first met on the set of That '70s Show, which they filmed from 1998 to 2006, but they didn't begin dating until April 2012 - after Ashton's split from first wife Demi Moore.
They married in July 2015, nine months after Wyatt was born. The pair are famously private, and rarely post pictures of themselves together or their daughter on social media.
Last month, Mila revealed just how much motherhood has changed her life. 'I re-prioritised my life, in all honesty,' she told Entertainment Tonight.
'I travelled through all my 20s, and I was very selfish, in a good way, not, like, in a way that I regret. In a great way. And I think having a kid made me realize how incredibly selfless I want to be.
'When we decided to have a kid, we were ready to no longer be selfish... Because innately, as humans, we're very selfish people, and we really wanted to have a family,' she said. 'It's definitely a learning process.'
Mila has been busy promoting her new film Bad Moms, a comedy about the realities of being a mum in which she appears alongside Kristen Bell. The film is due for release in July.