"They conceived through in vitro fertilization," a well-placed source within their camp tells Us. "They both desperately wanted more babies soon."
The chance of having fraternal twins at Angelina's age (33) naturally is under 1 percent; with in vitro, the chances are 25 percent.
Says Dr. Arthur Wisot of L.A.'s Reproductive Medical Group (who did not treat the couple), "We live in an era of reproductive freedom, so anybody can do anything they want within legal limits."
The actress chose the procedure (which can cost around $12,000 a pop) so "she wouldn't have to deal with the stress of trying to get pregnant," the source tells Us. "She could just knock it out."
Indeed, Jolie has spoken about her goal to do just that.
"If we're going to have 10 kids, we'd like to raise them while we're young," she told Elle U.K. last year. Brad Pitt turns 45 on December 18. A source adds: "They were too impatient." Knox and Vivienne — born in Nice, France on July 12 — join the couple's adopted brood of Maddox, 6, Pax, 4, Zahara, 3, and biological daughter Shiloh, 2.
Check out the new issue of Us Weekly, available now, for the complete story on why Brad pushed Angelina for biological kids, who inspired her to choose pregnancy over adoption this time around, and how they plan to raise their six young children.
Tell Us: How do you feel about using in vitro?
Sign up now for the Us Weekly newsletter to get breaking celebrity news, hot pics and more delivered straight to your inbox!