A new study by researchers at Johns Hopkins University shows that fulfilling unmet contraception demand by women in developing countries could reduce global maternal mortality by nearly a third, a potentially great improvement for one of the world's most vulnerable populations.

The study, published today in The Lancet, comes ahead of a major family planning conference in London organized by the British government and the Bill and Melinda Gates Foundation that is an attempt to refocus attention on the issue.

It has faded from the international agenda in recent years, overshadowed by efforts to combat Aids and other infectious diseases, as well as by ideological battles.

The proportion of international population assistance funds that went to family planning fell to 6 per cent in 2008, down from 55 per cent in 1995, while spending on HIV/Aids represented 74 per cent of the total in 2008, up from just 9 per cent in 1995, according to Rachel Nugent, a professor of global health at the University of Washington, who cited figures from the United Nations Population Fund.

But population growth has continued to surge, with the United Nations estimating last year that the world's population, long expected to stabilize, will instead keep growing.

Population experts warn that developing countries, particularly those in sub-Saharan Africa, where fertility continues to be high and shortages of food and water are worsening, will face deteriorating conditions if family sizes do not shrink.

"Family planning kind of faded from the radar screen, and now it is coming back," said John May, a visiting fellow at the Centre for Global Development and author of a book, "World Population Policies: Their Origin, Evolution, and Impact."

"There is a realisation from many different places that population issues are not going away," he said.

The issue of family planning is fraught in the United States, where government assistance often gets caught up in political battles. Contraception has again become controversial this political season, although the United States remains a major donor.

The Gates Foundation and the British government are pressing the issue. About $4 billion is expected to be pledged at the London conference to provide family planning services to 120 million women from the world's poorest countries over the next eight years.

"We hear time and again from women out in the field that they want the ability to plan their families," said Gary Darmstadt, director of family health at the Gates Foundation, who spoke by telephone from London. "We felt we needed to shine a light back onto the importance of this issue and get the conversation going."

Maternal deaths have declined dramatically since 1990, down by a third, according to the World Health Organization. But about 16 per cent 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 per cent 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 acknowledged, however, that maternal mortality record-keeping is weak in developing countries, a limitation of the study. They also found that an additional 29 per cent 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 Ms 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."

New York Times