An estimated 70,000 girls aged 15-19 and over 1 million infants born to young mothers die each year from complications related to childbearing, the report said.
"We looked at this lifetime risk of death, becoming a mother, but also things like the percent of women using modern contraception, the percentage of women who are literate, and attended school, whether women participate in national government," Save the Children's the reproductive health adviser, Mary Beth Powers, told CBS Radio News.
Some 99 percent of all maternal deaths occur in developing countries, he said.
Girls in sub-Saharan Africa face the highest risk rates for young mothers, Kiernan said. Outside Africa, the risks are especially high in Afghanistan, Bangladesh, India, Nepal, Yemen, Guatemala, Haiti and Nicaragua.
The group's annual "State of the World's Mothers" report ranks the well-being of mothers in 119 countries, taking into account their health, education and political status.
This year's report focused on mothers who are still children or adolescents, whose bodies often are not mature enough to handle the strains of pregnancy and childbirth.
While most of the study focused on girls in the developing world, it also reported on early motherhood in wealthy nations. Among industrialized nations, the United States has the largest number of teen pregnancies, followed by Russia and New Zealand.
The Republic of Korea has the lowest numbers of teen births, followed by Japan and the Netherlands.
The report found that education was the single biggest factor in helping girls postpone pregnancy and have healthier children.
In Nigeria, for example, research showed that only 7 percent of women with seven years of schooling gave birth before age 20, compared with 43 percent of those with no education, the report said.
That's also the problem in the U.S., said Mark Shriver, head of U.S. programs for Save The Children. Better after-school programs are needed for teen mothers.
"If they do learn basic reading skills and basic math skills, they're more likely to stay in school, they're more likely to get their high school diploma and move on to productive adulthood," he said.
The report's "Mothers' Index," which ranks the top 10 and the bottom 10 countries for mothers, shows a large literacy gap between the two extremes. In Sweden, ranked the best country for mothers, more than 99 percent of women are literate, while in Niger, ranked lowest, only 9 percent are.
"They have a very low chance of dying during pregnancy and childbirth" in Sweden, said Powers, while in Niger, "they have many childbirths, and each one is so risky."
After education, health care is key to addressing the problems of child motherhood, the report said. It recommended tailoring health services to newly married girls and first-time mothers, with voluntary family planning as a key component.
"Young married girls need access to contraception so they can delay first births until they are 20 when their bodies are more mature," Powers said.
Save the Children is part of a coalition of aid groups that will call upon the U.S. government during the G-8 Summit in June to commit $1 billion to global education by the year 2006, Powers said. Last year, the allocation was $326.5 million.
"We think the United States' fair share is triple that amount," she said.
After Sweden, the best countries for mothers are: Denmark, Finland, Austria, the Netherlands, Norway, Australia, Canada, the United Kingdom and the United States, ranked 10th.
"Clearly, the United States should be Number One in the way mothers are treated, so Number 10 is not acceptable," said Shriver.
After Niger, the lowest-ranking countries for mothers are: Burkina Faso, Ethiopia, Mali, Guinea-Bissau, Chad, Sierra Leone, Yemen, the Central African Republic and Mauritania.