Heavily pregnant Angelina Jolie today said that she couldn't imagine anything worse than losing a child.

The actress is in Cannes to talk about her latest role in Clint Eastwood's new film The Exchange, which has its world premiere tonight and is showing in competition at the festival.

The film is based on the real-life story of single mother Christine Collins (Jolie), whose nine-year-old son Walter goes missing.

Five months later a child claiming to be the boy is returned and police take the plaudits for reuniting mother and son.

Christine takes the boy home, but is convinced that he is not her son Walter.

The film is set in Los Angeles in the late 1920s and tells how the woman's invincible spirit brought down a police department and ushered in a new era of equality. She finds an ally in community activist Gustav Briegleb (John Malkovich).

In her search for answers, Christine becomes an unlikely heroine for those who have been swept aside.

Speaking today Jolie said: "To lose a child, I can't imagine anything worse, especially not knowing the fate of that child."

Jolie, who is pregnant with twins, said that as a modern, outspoken woman, she knew the part was going to be a challenge.

She said she imagined the frustration she would feel if she were in the situation.

She said: "I lost my own mother a few months before the film. My mother was very passive in many ways and very sweet, but when it came to her children, she was a lion."

She said Christine reminded her of her mother Marcheline Bertrand and it was a way of re-visiting her.

Jolie she had been nervous about working with Eastwood at first but said she found him to be a great all-round leader, giving respect to every member of the crew.

"I have never seen a director command such respect," she said.

Eastwood said that the theme of serial killing in the film was all the more devastating because of the added element of paedophilia.

He said: "Crimes against children are to me the most heinous crimes that there are. It never ceases to surprise you how cruel humanity can be. It's an important subject to take up but it's not going to be the last story like this in history."

Eastwood said the film demonstrated how one woman brought down corruption through her "tenacious attitude".

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