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 fertilisation. They both desperately wanted more babies soon."

Angelina, 33, only had a one per cent chance of conceiving fraternal twins naturally, but a 25 per cent chance by recipients of the treatment.

The Wanted actress was also keen to have the procedure - which can cost up to 12,000 U.S. dollars 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 fertilisation is a procedure where the egg cells are fertilised by sperm outside the woman's womb. The fertilised 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.