Angelina: I wanted biological child because of Brad

The couple have four children - five-year-old Maddox, who was adopted from Cambodia; two-year-old Zahara, who was adopted from Ethiopia; daughter, Shiloh, who was born to the couple last May, and Pax, 3, who was adopted from Vietnam in March.

The 31-year-old actress said: "I always said I was happy never to have a child biologically. (Brad) told me he hadn't given up that thought.

"Then, after Z came home (from Ethiopia), I saw Brad with her and Mad, and I realised how much he loved them, that a biological child would not in any way be a threat."

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Angelina Jolie says she changed her mind about having a biological child after seeing partner Brad Pitt with her two adopted children

Jolie wanted to adopt her latest child partly so that there was somebody around who was similar to her eldest child.

Asked if she adopted Pax so Maddox would have a brother who looks like him, Jolie told Reader's Digest that, after Shiloh's birth, "somebody in the house looked like Mommy and Daddy.

"It became clear to us that it might be important to have somebody around who is similar to the other children so they have a connection."

She went on: "We've tried to figure out a lot of private time for each of them. When everybody goes to bed, we give Mad time. When everybody is at school, we give Shiloh time. In between, Z and Pax each get special time."

She told the magazine, which is out on May 22, that they have a "big family sleep" on Sundays, when the children get in to bed with the couple and watch a film.

"We're talking about having to build a bigger bed," she said.

"Yesterday, picking up the kids from school, Brad turned around in the car, and there were three of (their kids). He couldn't stop laughing. We love them and are having a great time."