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Martha Ryan, 61

San Francisco

As a nurse in African refugee camps, Martha Ryan thought she'd seen it all. But returning to San Francisco in 1985, she was shocked to see women and children on the streets-a grim sign of the spike in the nation's homeless population. Ryan asked a pregnant woman if she was under a doctor's care, and her heart broke at the answer. "I can't worry about the baby in my belly," Ryan recalls her saying. "I'm worried about the kids I've got."

The exchange gave Ryan her life's mission. "Pregnancy," she remembers thinking, "is an amazing opportunity to help a woman turn her life around." And so was born the Homeless Prenatal Program (homelessprenatal.org), the nonprofit Ryan started in 1989 as a clinic in a homeless shelter. As homeless and needy women flocked there, the program offered services ranging from help with regular medical care and job training to substance-abuse and mental-health counseling.

Today, in a three-story building, with 200 volunteers and 68 employees, she has helped more than 2,000 women deliver healthy babies. One is Laura Close, 30, homeless and drug-addicted five years ago and now a college student raising Nathan, a lively 4-year-old. "Martha saved my life and my son's," Close says. "I don't want to think where we'd be without her."

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