The 30-year-old mom-to-be, who's expecting her first child with her former That 70's Show co-star, said she and Kutcher became more than friends "all of a sudden."
"One day, it just changed. All of a sudden, it wasn't the same. And I was really proud of myself for acknowledging that," she said.
Kunis added that when Kutcher proposed earlier this year, it was the "best day of my life."
"The best day of my life so far was the proposal. I cried. I was a mess. Not to discredit any relationships in my past, but this relationship is different," she said.
The Third Person star also said she "love[s] being pregnant," and knows exactly who should be in the delivery room when she gives birth.
"Two people are allowed in my delivery room: my doctor and my significant other," she said. "And [my future husband] is staying above the action. He'll be head to head. Not head to vag. Unless he wants to risk his life and see. But I wouldn't if I were him."