UNITED NATIONS (Reuters) - The United Nations program on HIV/AIDS (UNAIDS) on Thursday named fashion designer and former Spice Girl Victoria Beckham a goodwill ambassador.

Beckham, who said she was inspired to help after a "life-changing" visit to HIV clinics in South Africa, will focus on working toward ensuring that all children are born free from HIV and that children and women who are living with and affected by HIV have access to medicines and care, UNAIDS said.

"It's taken me getting to 40 years old to realize I have a responsibility as a woman, as a mother, I have a voice that people will listen to," she told a news conference at the United Nations on the sidelines of the U.N. General Assembly.

"I'm not going to sit here and pretend to know everything right now, I don't, I'm learning," said Beckham, adding that she planned to take field trips to learn more about the problem and how she could help.

Sub-Saharan Africa is the region hardest hit by HIV, with 24.7 million HIV-positive people in 2013. Women account for 58 percent of those with HIV in the region, which is also home to 85 percent of pregnant women with HIV, according to UNAIDS.

Last month Beckham auctioned off 600 pieces of clothing, including several evening dresses, to raise money and awareness for mothers living with HIV in sub-Saharan Africa.

UNAIDS said that in 2013, one third of pregnant women living with HIV did not have access to the life-saving medicines and some 240,000 children became infected with HIV. But in the past five years access to antiretroviral medicines for pregnant women with HIV helped 900,000 children to be born without HIV.