By Michelle Goldberg, Newsweek

In the 12 years since Melinda Gates and her husband, Bill, created the Gates Foundation, the world’s largest philanthropic organization, she has done a lot of traveling. A reserved woman who has long been wary of the public glare attached to the Gates name, she comes alive, her associates say, when she’s visiting the foundation’s projects in remote corners of the world. “You get her out in the field with a group of women, sitting on a mat or under a tree or in a hut, she is totally in her element, totally comfortable,” says Gary Darmstadt, director of family health at the foundation’s global health program.

Visiting vaccine programs in sub-Saharan Africa, Gates would often ask women at remote clinics what else they needed. Very often, she says, they would speak urgently about birth control. “Women sitting on a bench, 20 of them, immediately they’ll start speaking out and saying, ‘I wish I had that injection I used to get,’” says Gates. “‘I came to this clinic three months ago, and I got my injection. I came last week, and I couldn’t get it, and I’m here again.’”

They were talking about Depo-Provera, which is popular in many poor countries because women need to take it only four times a year, and because they can hide it, if necessary, from unsupportive husbands. As Gates discovered, injectable contraceptives, like many other forms of birth control, are frequently out of stock in clinics in the developing world, a result of both funding shortages and supply-chain problems.

Women would tell her that they’d left their farms and walked for hours, sometimes with children in tow, often without the knowledge of their husbands, in their fruitless search for the shot. “I was just stunned by how vociferous women were about what they wanted,” she says.

Because of those women, Gates made a decision that’s likely to change lives all over the world. As she revealed in an exclusive interview with Newsweek, she has decided to make family planning her signature issue and primary public health a priority. “My goal is to get this back on the global agenda,” she says. She is sitting in an office in the Gates Foundation’s 900,000-square-foot headquarters in downtown Seattle, a pair of airy boomerang-shaped buildings flooded with natural light. It was here at headquarters late last year that she announced her new emphasis on contraception at an all-staff meeting, to thrilled applause.

Now the foundation, which is worth almost $34 billion, is putting her agenda into practice. In July it’s teaming up with the British government to cosponsor a summit of world leaders in London, to start raising the $4 billion the foundation says it will cost to get 120 million more women access to contraceptives by 2020. And in a move that could be hugely significant for American women, it is pouring money into the long-neglected field of contraceptive research, seeking entirely new methods of birth control. Ultimately Gates hopes to galvanize a global movement. “When I started to realize that that needed to get done in family planning, I finally said, OK, I’m the person that’s going to do that,” she says.

Despite Gates’s passion, stepping forward wasn’t an easy decision. For one thing, the former Microsoft manager has always shunned the spotlight. The first time she agreed to a magazine profile was in 2008, 14 years after her marriage, when she spoke to Fortune about the foundation’s work. “I was reluctant to speak out on behalf of any foundation issues early on, because I had little kids, and I wanted some privacy in my family life,” she says.

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