Angelina Jolie's Twins Conceived Through In Vitro

Angelina Jolie's new born twins Knox and Vivienne were conceived through in vitro (IVF) treatment, it has been revealed.

According to American magazine US Weekly, Jolie, 33, turned to fertility treatments in order to conceive quicker.

A source tells the publication, "They conceived through in vitro fertilization

"They both desperately wanted more babies soon."

The chance of having fraternal twins naturally at Angelina's age is under 1 percent; with in vitro, the chances are 25 percent. Says Dr. Arthur Wisot of L.A.'s Reproductive Medical Group (who did not treat the couple), "We live in an era of reproductive freedom, so anybody can do anything they want within legal limits."

The oldest patient reported to deliver after IVF was Omkari Panwar, a 70 year old woman from India who delivered a set of twins in 2008.

The Tomb Raider star chose the procedure -- which can cost around $12,000-a-time -- so "she wouldn't have to deal with the stress of trying to get pregnant," the source said. "She could just knock it out."

Jolie -- who also has three adopted children, Maddox, Pax, and Zahara, as well as her eldest biological child, Shiloh, with partner Brad Pitt -- recently spoke of her desire to have a large family at a young age.

She told Elle U.K., "If we're going to have 10 kids, we'd like to raise them while we're young."

A source adds, "They were too impatient."