Angelina Jolie and Brad Pitt turned to in vitro fertilization to conceive twins Vivienne and Knox, US Weekly reports.

A source close to the couple told the magazine that "they both desperately wanted more babies soon," and says that the actress, 33, chose the $12,000 procedure so "she wouldn't have to deal with the stress of trying to get pregnant. She could just knock it out."

Dr. Arthur Wisot of L.A.'s Reproductive Medical Group, who did not treat the couple, told US Weekly that "we live in an era of reproductive freedom, so anybody can do anything they want within legal limits."

Last year, the "Wanted" star told Elle U.K. that "if we're going to have 10 kids, we'd like to raise them while we're young." The chance of having fraternal twins at Angelina's age naturally is under 1 percent, but with in vitro, the chances rise to 25 percent.

Vivienne Marcheline and Knox Leon, born in France on July 12, joined siblings Maddox, 6, Pax, 4, Zahara, 3, and biological daughter Shiloh, 2.

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