She says she changed her mind while watching Pitt bond with adopted children Maddox and Zahara.
"He stopped talking about having other kids in the other way," she says in the magazine's November issue. "So I suppose in my heart I realized he was happy with them as his children, completely. I knew he would never see them as different, and that gave me a certain peace."
After that revelation, "I suppose I just looked at him and loved him and just felt open to (getting pregnant). I suddenly wanted to. It's one of those things you can't explain," she says.
The couple welcomed daughter Shiloh in May 2006 and twins Knox Leon and Vivienne Marcheline in July.
"Now I wouldn't trade that experience for the world," Jolie says. "It taught me a lot about life, just the process of it, and now we have three other beautiful children that wouldn't otherwise be here."
The 33-year-old actress is photographed apparently breast-feeding on the cover of W. The black-and-white photo shows a smiling Jolie in a sleeveless top, which has been opened to reveal part of her left breast and a tiny hand ostensibly belonging to one of her twins.
The cover headline promises exclusive "private photos" taken of Jolie by Pitt, with whom she is also parent to 4-year-old Pax, adopted from Vietnam.
"I'm with a man who's evolved enough to look at my body and see it as more beautiful, because of the journey it has taken and what it has created. He genuinely sees it that way. So I genuinely feel even sexier," says Jolie, who stars in Clint Eastwood's upcoming missing-child drama "Changeling."
Jolie, who divorced Billy Bob Thornton and Jonny Lee Miller, calls Pitt the "right man" for her.
"I have a lot of respect for him, and he helps me to be better and fight hard for things that I love," she says.