She's a 40-year-old mother of eight, with a ninth child due soon. The family homestead in a Burundi village is too small to provide enough food, and three of the children have quit school for lack of money to pay required fees.

"I regret to have made all those children," says Godelive Ndageramiwe. "If I were to start over, I would only make two or three."

At Ahmed Kasadha's prosperous farm in eastern Uganda, it's a different story.

"My father had 25 children - I have only 14 so far, and expect to produce more in the future," says Kasadha, who has two wives. He considers a large family a sign of success and a guarantee of support in his old age.

By the time Ndageramiwe's ninth child arrives, and any further members of the Kasadha clan, the world's population will have passed a momentous milestone. As of Oct. 31, according to the U.N. Population Fund, there will be 7 billion people sharing Earth's land and resources.

In Western Europe, Japan and Russia, it will be an ironic milestone amid worries about low birthrates and aging populations. In China and India, the two most populous nations, it's an occasion to reassess policies that have already slowed once-rapid growth.

But in Burundi, Uganda and the rest of sub-Saharan Africa, the demographic news is mostly sobering as the region staggers under the double burden of the world's highest birthrates and deepest poverty. The regional population of nearly 900 million could reach 2 billion in 40 years at current rates, accounting for about half of the projected global population growth over that span.

"Most of that growth will be in Africa's cities, and in those cities it will almost all be in slums where living conditions are horrible," said John Bongaarts of the Population Council, a New York-based research organization.

Is catastrophe inevitable? Not necessarily. But experts say most of Africa - and other high-growth developing nations such as Afghanistan and Pakistan - will be hard-pressed to furnish enough food, water and jobs for their people, especially without major new family-planning initiatives.

"Extreme poverty and large families tend to reinforce each other," says Lester Brown, the environmental analyst who heads the Earth Policy Institute in Washington. "The challenge is to intervene in that cycle and accelerate the shift to smaller families."

Without such intervention, Brown says, food and water shortages could fuel political destabilization in developing regions.

"There's quite a bit of land that could produce food if we had the water to go with it," he said. "It's water that's becoming the real constraint."

The International Water Management Institute shares these concerns, predicting that by 2025 about 1.8 billion people will live in places suffering from severe water scarcity.

According to demographers, the world's population didn't reach 1 billion until 1804, and it took 123 years to hit the 2 billion mark in 1927. Then the pace accelerated - 3 billion in 1959, 4 billion in 1974, 5 billion in 1987, 6 billion in 1998.

Looking ahead, the U.N. projects that the world population will reach 8 billion by 2025, 10 billion by 2083. But the numbers could be much higher or lower, depending on such factors as access to birth control, infant mortality rates and average life expectancy - which has risen from 48 years in 1950 to 69 years today.

"Overall, this is not a cause for alarm - the world has absorbed big gains since 1950," said Bongaarts, a vice president of the Population Council. But he cautioned that strains are intensifying: rising energy and food prices, environmental stresses, more than 900 million people undernourished.

"For the rich, it's totally manageable," Bongaarts said. "It's the poor, everywhere, who will be hurt the most."

The executive director of the U.N. Population Fund, former Nigerian health minister Babatunde Osotimehin, describes the 7 billion milestone as a call to action - especially in the realm of enabling adolescent girls to stay in school and empowering women to control the number of children they have.

"It's an opportunity to bring the issues of population, women's rights and family planning back to center stage," he said in an interview. "There are 215 million women worldwide who need family planning and don't get it. If we can change that, and these women can take charge of their lives, we'll have a better world."

But as Osotimehin noted, population-related challenges vary dramatically around the world. Associated Press reporters on four continents examined some of most distinctive examples:

THE ASIAN GIANTS

It's 6 p.m. in Mumbai, India's financial hub, and millions of workers swarm out of their offices, headed to railway stations for a ride home. Every few minutes, as a train enters the station, the crowd surges forward.

For nearly 7 million commuters who ride the overtaxed suburban rail network each work day, every ride is a scramble. Each car is jam-packed; sometimes, riders die when they lose their foothold while clinging to the doors.

Across India, the teeming slums, congested streets, and crowded trains and trams are testimony to the country's burgeoning population. Already the second most populous country, with 1.2 billion people, India is expected to overtake China around 2030 when its population soars to an estimated 1.6 billion.

But even as the numbers increase, the pace of the growth has slowed. Demographers say India's fertility rate - now 2.6 children per woman - should fall to 2.1 by 2025 and to 1.8 by 2035.

More than half of India's population is under 25, and some policy planners say this so-called "youth dividend" could fuel a productive surge over the next few decades. But population experts caution that the dividend could prove to be a liability without vast social investments.

"If the young population remains uneducated, unskilled and unemployable, then that dividend would be wasted," says Shereen Jejeebhoy, a Population Council demographer in New Delhi.

Population experts also worry about a growing gender gap, stemming largely from Indian families' preference for sons. A surge in sex-selection tests, resulting in abortion of female fetuses, has skewed the ratio, with the latest census showing 914 girls under age 6 for every 1,000 boys.

Family planning is a sensitive issue. In the 35 years since one government was toppled for pursuing an aggressive population control program, subsequent leaders have been reluctant to follow suit.

For now, China remains the most populous nation, with 1.34 billion people. In the past decade it added 73.9 million, more than the population of France or Thailand.

Nonetheless, its growth has slowed dramatically and the population is projected to start shrinking in 2027. By 2050, according to some demographers, it will be smaller than it is today.

"It's like a train on the track that's still moving but the engine is already off," says Gu Baochang, a professor of demography at Beijing's Renmin University.

WESTERN EUROPE AND THE U.S.

Spain used to give parents 2,500 euros (more than $3,000) for every newborn child to encourage families to reverse the country's low birth rate. But the checks stopped coming with Spain's austerity measures, raising the question of who will pay the bills to support the elderly in the years ahead.

It's a question bedeviling many European countries which have grappled for years over how to cope with shrinking birth rates and aging populations - and are now faced with a financial crisis that has forced some to cut back on family-friendly government incentives.

Spain and Italy, both forced to enact painful austerity measures in a bid to narrow budget deficits, are battling common problems: Women have chosen to have their first child at a later age, and the difficulties of finding jobs and affordable housing are discouraging some couples from having any children at all.

In 2010, for the fourth consecutive year, more Italians died than were born, according to the national statistics agency. Italy's population nonetheless grew slightly to 60.6 million due to immigration, which is a highly charged issue across Europe.

Italy's youth minister Giorgia Meloni said earlier this year that measures to reverse the birth rate require "millions in investment" but that the resources aren't available.

Unlike many countries in Europe, France's population is growing slightly but steadily every year. It has one of the highest birth rates in the European Union with around 2 children per woman.

One reason is immigration to France by Africans with large-family traditions, but it's also due to family-friendly legislation. The government offers public preschools, subsidies to all families that have more than one child, generous maternity leave, and tax exemptions for employers of nannies.

Like France, the United States has one of the highest population growth rates among industrialized nations. Its fertility rate is just below the replacement rate of 2.1 children per woman, but its population has been increasing by almost 1 percent annually due to immigration. With 312 million people, the U.S. is the third most populous country after China and India.