"They conceived through in vitro fertilization," Usmagazine.com quoted a well-placed source, as saying.
"They both desperately wanted more babies soon," the source added.
The source further added that 'Tom Raider' star chose the procedure, "as the she wouldn't have to deal with the stress of trying to get pregnant."
Knox and Vivienne were born in Nice, France on July 12. The couple already has three adopted kids Maddox, 6, Pax, 4, Zahara, 3, and a biological daughter Shiloh, 2.