ANGELINA Jolie and Brad Pitt reportedly conceived their twins using in vitro fertilization. The couple - who welcomed Knox Leon and Vivienne Marcheline into the world on July 12 - were reportedly so desperate to add to their ever-expanding brood they turned to IVF infertility treatment in a bid to conceive. A source close to the couple told Us Weekly magazine: "They conceived through in vitro fertilization. They both desperately wanted more babies soon." Angelina, 33, only had a one percent chance of conceiving fraternal twins naturally, but a 25 percent chance by recipients of the treatment. The 'Wanted' actress was also keen to have the procedure - which can cost up to $12,000 a time - because she didn't want to have any problems conceiving. The source added: "She wouldn't have to deal with the stress of trying to get pregnant. They were impatient." In vitro fertilization is a procedure where the egg cells are fertilized by sperm outside the woman's womb. The fertilized egg, called a zygote, is then transferred to the patient's uterus. Angelina and Brad also have three adopted children, Maddox, six, four-year-old Pax, Zahara, three, and two-year-old biological daughter Shiloh.