Maternal deaths have declined dramatically since 1990, down by a third, according to the World Health Organization.

But about 16 percent of the world’s population lives in countries where fertility is still more than four children per woman, Mr. May said. The numbers of people are expected to more than triple in these places during this century, an issue that is urgent not only for their economies and environment, but also for the women themselves, who women’s rights advocates argue would benefit from more power to decide about bearing children.

The Lancet study, which the Gates Foundation financed, draws on maternal mortality and survey data from the United Nations and World Health Organization to estimate the annual number of maternal deaths in 172 countries and the share that could be preventable by the use of contraception.

Birth control reduces health risks, the researchers said, by delaying first pregnancies, which carry higher risks in very young women; cutting down on unsafe abortions, which account for 13 percent of all maternal deaths in developing countries; and controlling dangers associated with pregnancies that are too closely spaced.

The authors of the Lancet study, researchers at the Bloomberg School of Public Health at Johns Hopkins, found that the number of maternal deaths in those countries in 2008 would have nearly doubled without contraception. They acknowleged, however, that maternal mortality record-keeping is weak in developing countries, a limitation of the study. They also found that an additional 29 percent of the deaths could have been prevented if women who wanted birth control would have received it, a concept called unmet need that is estimated using surveys of mothers in developing countries.

Even so, simply providing contraceptives to people who are not using them and who say they want to avoid pregnancy might not be enough to actually do so, and Professor Nugent said the study’s conclusions might be optimistic.

The lack of birth control in poor countries has become an important issue for Melinda Gates, who argued in highly personal remarks in April that birth control should not be controversial, because it improves women’s lives.

“Somewhere along the way we got confused by our own conversation and we stopped trying to save these lives,” she said. She added: “We’re not talking about abortion. We’re not talking about population control. What I’m talking about is giving women the power to save their lives.”