In India alone, by 2100 there may be 600 million fewer people than predicted.
The decline in birthrates in nations where poverty and illiteracy are still widespread defies almost all conventional wisdom. Planners once argued — and some still do — that falling birthrates can only follow improved living standards and more educational opportunities, not outrun them. It now seems that women are not waiting for that day.
A few demographers are venturing to say that the trend may have little to do with government policies on family planning or foreign aid.
Since the United Nations conference on population and development in Cairo in 1994, women in many countries have said that if they had control over their reproductive lives, lower fertility rates would be a given. Women's health organizations now say that is happening.
'From Delhi to Rio, women's health advocates have stood fast against top-down population policies, and have stood for women's rights — and abilities — to make decisions about their bodies,' said Cynthia Steele, vice president for programs at the International Women's Health Coalition in New York. 'Whether they live in villages or high-rises, women have always known what's best for them and their families. Now we're seeing the results of their own choices to have fewer children.'
Joseph Chamie, the director of the United Nations population division, said: 'A woman in a village making a decision to have one or two or at most three children is a small decision in itself. But when these get compounded by millions and millions and millions of women in India and Brazil and Egypt, it has global consequences.'
Mr. Chamie said it had been assumed that the fertility rates in big developing countries — the number of births, on average, per woman — would fall at best only to what is known as replacement level. That number is 2.1, or a little more than one child for each parent. But in big countries, even that pace would add a huge number to an already large population base before the trend eventually moderates.
Demographers may now be willing to go out on a limb and say that the fertility rates in the big developing countries may even drop below the replacement level, and sooner than most of them would have thought possible.
That would follow the trend already established in industrial countries, where the population slowdown has caused concerns about shrinking labor forces and aging populations.
Just as women are pushing for a larger role in economic life around the world, they are also apparently becoming more assertive within families. 'We're breaking both the fertility floor and the glass ceiling,' Mr. Chamie said.
In India, Gita Sen, professor of economics at the Indian Institute of Management in Bangalore, said in a telephone interview that there were important cultural factors at work.
'Fertility in India is declining and it is declining faster than many people had expected,' she said. One reason, she said, is 'that with increasing awareness on the part of women, they are being able to control their own fertility much better.'
'It seems to start in one village and then spread to other places around that area,' she said. 'Attitudes are changing, and people are watching what their neighbors are doing.'
With declining infant mortality, mothers become more confident that their babies will survive, Ms. Sen added, and so they can have fewer children. She and other experts say that urbanization also eases some family controls on women, and makes contraceptive pills or devices easier to find.
Both family pressure and lack of access to reproductive health care limit many women.
Ms. Sen said a family survey in 1999 in India, where the fertility rate is still about 3.0 per woman, underlined the change in attitudes. 'It was a very detailed survey that interviewed close to 90,000 married women all across the country,' she said. 'One of the most striking things in that was that even in the poorer northern states if you ask women about the number of children that they want, it's much lower than the number that they actually have.'
In Brazil, women have reduced fertility levels without an official national family planning policy, Ana Maria Goldani of the department of sociology and Latin American studies at U.C.L.A. wrote in a paper for next week's conference. Brazil's fertility rate has tumbled, to 2.27 from 6.15 in the last half century, and it continues to fall for reasons that Ms. Goldani says are only now being analyzed.
Gelson Fonseca, Brazil's ambassador to the United Nations, said that television was important. Brazilians see small and apparently happy families in television programs and think about emulating that example.
In Bangladesh, family planning experts noticed a decade ago that in some of the remotest areas, information gleaned from satellite television was influencing contraceptive choices. In one case, a certain intrauterine device was rejected by many women in an area where one of them had seen it described as hazardous in a Western television program.
There are 74 countries in what the United Nations calls the intermediate-level fertility group, with births between 2.1 and 5 per woman.
This group includes very populous countries like Bangladesh, Brazil, Egypt, India, Indonesia, Iran, Mexico, the Philippines and Vietnam.
Some demographers question whether any one trend will fit them all, and ask whether it may not be as mistaken to herald a general population decline to below replacement levels as it was to pronounce that the larger developing nations would never reach this stage.
John C. Caldwell of the Australian National University urges caution. In a paper prepared for next week's meeting, he writes of a 'loss of fervor' in the developing world for further fertility decline.
Countries are not homogenous, he argued, and there are some large ones in Africa and Asia where there will continue to be a preference for more children.
But Ms. Sen says that she is not concerned about India backsliding in the movement toward ever-lower fertility rates, arguing that education and cultural factors are becoming catalysts for change.
'For a very long time we've had a huge problem in terms of 50 to 60 percent of the female population being illiterate,' she said. 'The most recent census, the 2001 census, shows the biggest increases in literacy happening in some of the poor northern states — big jumps in literacy — and that means girls going to school.
'Those same girls are going to be making the fertility decisions in another 10 years or so,' she said, 'and I don't think they are going to make them in the same way that their illiterate mothers may have.'