Brad Pitt and Angelina Jolie used in vitro fertilisation to conceive twins Knox and Vivienne, according to an American report.

The couple wanted to expand their family quickly rather than waiting for nature to take its course, a source told Us Weekly.

And that could be why they have a double bundle of joy - the chances of a 33-year-old woman like the Wanted star having fraternal twins naturally are fewer than one per cent, while with in-vitro raises the likelihood to one in four, according to the publication.

Deserpate for more babies: Brad Pitt and Angelina Jolie used in vitro fertilisation to conceive twins, according to a US report

'They conceived through in vitro fertilisation,' a source described as being close to Jolie and Pitt, 44, said. 'They both desperately wanted more babies soon.'

They chose the US$12,000 procedure so 'she wouldn't have to deal with the stress of trying to get pregnant,' the source added. 'She could just knock it out.'

'Things were so hectic with their filming schedules and the kids, the simply didn't have time to keep trying. They were too impatient.'

Knox and Vivienne were born in Nice on July 12, joining the couple's adopted children, Maddox, six, Pax, four and three-year-old Zahara, who are adopted, and their biological daughter Shiloh, who was born in Namibia two years ago.

Enlarge 

Expanding family: Angelina and Brad now have twins to add to their brood, which includes Maddox, Shiloh, Zahara and Pax

Jolie has spoken in the past about wanting to have her family quickly, telling a magazine last year: 'If we're going to have 10 kids, we'd like to raise them while we're young.'

Another source told Us: 'They were too impatient.'

But Dr Arthur Wisot of the Reproductive Medical Group in Los Angeles, who did not treat the couple, said: 'We live in an era of reproductive freedom, so anybody can do anything they want within legal limits.

Oscar-winning Girl, Interrupted star Jolie will next be seen in The Changeling, opening in January and Pitt has Burn After Reading out in October.