Showbiz
Rex Features
The couple, who welcomed new arrivals Knox Leon and Vivienne Marcheline on July 12, are said to have undergone fertility treatment to fall pregnant as quickly as possible.
A source told Us Weekly: "They conceived through in vitro fertilization. They both desperately wanted more babies soon."
An expert is quoted as claiming that the chances of a 33-year-old conceiving fraternal twins naturally is less than 1%, with IVF boosting the odds to 25%.
Pitt and Jolie have another biological daughter and three adopted children.
> Pitt threatens legal action on family snaps
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