Media playback is unsupported on your device Media caption The BBC's Fergus Walsh explains the background to Angelina Jolie's decision

Hollywood actress Angelina Jolie has undergone a double mastectomy to reduce her chances of getting breast cancer.

The 37-year-old mother of six has explained her reasons for having the surgery in the New York Times.

She said her doctors estimated she had an 87% risk of breast cancer and a 50% risk of ovarian cancer. "I decided to be proactive and to minimise the risk as much I could," she wrote.

Her partner, Brad Pitt, praised her choice as "absolutely heroic".

Risk reduced

Ms Jolie said the process began in February and was completed by the end of April.

In an article entitled My Medical Choice, she explained that her mother fought cancer for nearly a decade and died at the age of 56.

Family history of breast cancer About one in five women diagnosed with breast cancer will have a significant family history of the disease

Having close family members - first-degree relatives like a sister, mother, aunt or uncle - with breast cancer raises a woman's own risk of the disease

Certain genes such as BRCA1 and BRCA2 are strongly linked with breast cancer and can be detected with genetic tests

Women carrying the BRCA1 mutation have up to a 65% chance of developing breast cancer by the time they are 70

Fewer than 1% of women are at high risk of developing breast cancer

Another 2% of women deemed to be at moderate risk might also benefit from taking preventive breast cancer drugs for five years, say experts DNA mapping for cancer patients Breast cancer: drugs or surgery?

She said she had sought to reassure her children that the same illness would not take her away from them, "but the truth is I carry a 'faulty' gene, BRCA1, which sharply increases my risk of developing breast cancer and ovarian cancer".

She said that once she "knew that this was my reality", she had taken the decision to undergo the nine weeks of complex surgery required to have a double mastectomy, followed by reconstruction of the breasts with implants.

"There have been many advances in this procedure in the last few years, and the results can be beautiful," she wrote.

Media playback is unsupported on your device Media caption Dr Kristi Funk said she hoped Angelina Jolie's choice would help raise awareness around the world

Her chances of developing breast cancer have now dropped from 87% to under 5%, she said.

She praised her partner, actor Brad Pitt, for his love and support throughout the procedure, and said she was reassured that her children had found nothing in the results "that makes them uncomfortable".

"I feel empowered that I made a strong choice that in no way diminishes my femininity," she said.

"For any woman reading this, I hope it helps you to know you have options," Ms Jolie went on to say.

"I want to encourage every woman, especially if you have a family history of breast or ovarian cancer, to seek out the information and medical experts who can help you through this aspect of your life, and to make your own informed choices."

In an interview with the Evening Standard, Brad Pitt said: "Having witnessed this decision firsthand, I find Angie's choice, as well as so many others like her, absolutely heroic.

"All I want for is for her to have a long and healthy life, with myself and our children. This is a happy day for our family."

Ms Jolie, an award-winning actress and director, is also a long-time supporter of humanitarian causes. She is currently a special envoy for the UN Refugee Agency.

During the period she was undergoing the double mastectomy procedure, Ms Jolie visited the Democratic Republic of Congo with UK Foreign Secretary William Hague and attended the G8 summit of foreign ministers in London to raise awareness over sexual violence in conflict.

She also helped launch a charity to fund girls' education set up by the Pakistani schoolgirl activist Malala Yousafzai, who survived being shot by the Taliban last October.

Ms Jolie has three biological children and three adopted children.

Risk factor

Emma Parlons, a 38-year-old mother from London who had the operation three years ago, welcomed her decision to raise awareness of the issue.

Media playback is unsupported on your device Media caption Emma Parlons: "If somebody said your flight was 86% likely to come down, you wouldn't get on that plane"

Ms Parlons said her risk of getting breast cancer was the same as the actress's. "If somebody said your flight across the Atlantic was 86% likely to come down, you wouldn't get on that plane would you?" she said in an explanation of her reasons for having the operation.

Professor Gareth Evans, of the Manchester Breast Centre in Britain, said the two genes BRCA1 and BRCA2 (breast cancer one and two) "were the first two majorly breast cancer pre-disposing genes that were identified" and are also linked to an increased risk of ovarian cancer.

Angelina Jolie Born in 1975 to actors Jon Voight and Marcheline Bertrand

Has spoken of a troubled childhood, in which she self-harmed and took drugs

Won an Oscar for best-supporting actress in 2000

Has visited conflict zones and raised awareness of refugees and sexual violence

Granted honorary citizenship of Sarajevo for 2011 film about Bosnian war

Has three adopted children and three biological children with partner Brad Pitt Profile: Angelina Jolie

"The risk associated with the genes isn't simply an exact figure like 87%," he told the BBC.

"It does depend on other risk factors, and so the risk for someone with a BRCA1 mutation could vary anywhere between 50 and 95% for breast cancer."

Currently, women facing a strong likelihood of developing breast cancer have only two real options - to have both of their breasts removed (a double mastectomy) or hope that it will never actually happen.

In January, the drugs watchdog in England and Wales, the National Institute for Health and Clinical Excellence, recommended women at a high genetic risk of breast cancer should be given the option of taking the drug Tamoxifen, or another one called raloxifene, for five years to cut their lifetime risk of the disease.