The pair have been good friends for years after starring in That 70s Show together, and Mila added that: "One day, it just changed."
"All of a sudden, it wasn't the same. And I was really proud of myself for acknowledging that," the actress said.
Ashton Kutcher and Mila Kunis
The pair began dating in 2012 and in January 2014 Ashton proposed before they announced in March they were expecting their first child.
Speaking to Marie Claire of the proposal, Mila said: "The best day of my life so far was the proposal. I cried. I was a mess."
The 30-year-old also revealed that she has already made plans for the baby's delivery, and that there will only be two people in the room - Ashton and the doctor.
Two people are allowed in my delivery room," she shared. "My doctor and my significant other. And he is staying above the action... Unless he wants to risk his life and see."