Angelina Jolie and partner Brad Pitt used fertility treatments to conceive their newborn twins, a new report has suggested.The couple were desperate to quickly expand their brood - and turned to artificial insemination so they wouldn't have to "deal with the stress" of trying to get pregnant, reports Us Weekly magazine.A source tells the publication, "They conceived through in vitro fertilisation. They both desperately wanted more babies soon."Last year, the Tomb Raider star told U.K. magazine Elle: "If we're going to have 10 kids, we'd like to raise them while we're young."Fraternal twins Knox Leon and Vivienne Marcheline - born this month in Nice, France - became the fifth and six children to join the Pitt-Jolie clan of Maddox, six, Pax, four, Zahara, three, and biological daughter Shiloh, two.