BEIJING, July 29 -- Hollywood celebs Angelina Jolie and Brad Pitt reportedly conceived their newborn twins using in vitro fertilisation. 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.

US actress Angelina Jolie is seen at the Cannes Film Festival in May 2008. Hollywood celebs Angelina Jolie and Brad Pitt reportedly conceived their newborn twins using in vitro fertilisation.

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.