A representative for the couple has confirmed that Mila is pregnant and the loved-up pair, who already have 20-month-old daughter Wyatt together, are excited to become parents once more..
A source told PEOPLE: "Ashton genuinely lights up when talking about Wyatt and the little one on the way. Everything about being a father is natural to him."
In May, Mila admitted she and Ashton were trying for another baby during an appearance on 'The Ellen DeGeneres Show'.
When asked by Ellen if she was currently "trying" for a baby, the 32-year-old actress replied: "Like am I having sex, Ellen? Yes."
Ashton - who was previously married to Demi Moore and acted as stepfather to her three daughters, Rumer, Scout and Tallulah, with Bruce Willis - has described parenthood as the greatest thing ever.
'The Ranch' star said: "It's legitimately the greatest thing ever."