LOS ANGELES, California (X17online) - Angelina Jolie and Brad Pitt went with costly in vitro fertilization to conceive twins Knox Leon and Vivienne Marcheline, according to Us Weekly. A source told the magazine, "They conceived through in vitro fertilization. They both desperately wanted more babies soon." The source added that the couple chose in vitro so Jolie "wouldn't have to deal with the stress of trying to get pregnant."

In addition to the "stress" of getting pregnant, the couple may have had difficulty conceiving in the first place. Pitt is 44 years old, and the chance of someone having fraternal twins at Angelina's age (33) without fertility treatments is under 1 percent, but with in vitro, the chances shoot up to 25 percent.

CLICK HERE to see all of X17online's stories on Angelina Jolie

CLICK HERE to see all of X17's Angelina Jolie videos

CLICK HERE to see all of X17online's stories on Brad Pitt

CLICK HERE to see all of X17's Brad Pitt videos