Angelina Jolie and Brad Pitt's newborn twins were conceived via in vitro fertilisation, according to a new report.

The superstar couple are said to have opted for fertility treatment because they didn't have time to conceive naturally.

"They conceived through in vitro fertilization," an insider tells Us Weekly.

"They both desperately wanted more babies soon."

Angelina gave birth to twins Knox Leon and Vivienne Marcheline in a Nice hospital on July 12, and, according to an expert quoted by the rag, the chance of having fraternal twins at the age of 33 naturally is under 1 percent.

In vitro reportedly increases the chances to 25 percent.

The report goes on to claim that the actress opted for in vitro so "she wouldn't have to deal with the stress of trying to get pregnant. She could just knock it out."

"They were too impatient."