The couple, who got engaged earlier this year, have decided to wait until after the 'Black Swan' star gives birth to their first child in October to get married.
A source said: "[They're] not planning to get married before the baby is born."
The duo - who are reportedly expecting a baby girl - recently purchased a new $10 million child-friendly mansion in Beverly Hills, which offers plenty of privacy, after the 'Two and a Half Men' actor, 36, put his bachelor pad on the market for $12 million.
The insider also told E! News: "They have a lot going on right now. They've been focused on prepping for the baby and the new house.
"But they are really happy and in love. They can't wait for their baby to arrive."
The couple - who recently returned from a romantic babymoon in French Polynesia - were originally planning to tie the knot a registry office in Los Angeles so Mila could focus on her pregnancy, but the 30-year-old actress previously admitted being married wasn't important to her.
She said: "Not to say that I don't believe in it, but it's just not something that's important to me.
"But I will have children. I'm too selfish to have them now, but when I do, I don't feel like I need to be married."
Ashton began dating the brunette beauty in 2012, following his split from his first wife Demi Moore, but they have known each other since Mila was 14.