Actress Angelina Jolie arrives at a photocall for the film "The Exchange" during the 61st Annual Cannes Film Festival in Cannes, France on May 20, 2008. (UPI Photo/David Silpa)

Knox and Vivienne, the couple's fifth and sixth children, were born July 12 in France.

"They conceived through in vitro fertilization," a source close to Jolie and Pitt told the magazine. "They both desperately wanted more babies soon."

The odds of conceiving fraternal twins naturally at Jolie's age, 33, is less than 1 percent, Usmagazine.com noted.

The Oscar-winning actress opted for the in vitro procedure so "she wouldn't have to deal with the stress of trying to get pregnant," the insider told the magazine. "She could just knock it out."