As the U.S. population speeds toward 300 million, the growth is producing headaches for Americans fed up with traffic congestion, sprawl and dwindling natural resources.

But the alternatives are pretty scary, too. Just look at Europe and Japan, which are on the verge of such big population losses that several countries are practically begging women to have babies.

"Europe and Japan are now facing a population problem that is unprecedented in human history — declining population over time with an increase in the percentage of old people," said Bill Butz, president of the Population Reference Bureau, a Washington think tank.

Countries have lost people because of wars, disease and natural disasters but never — at least in modern history — because women stopped having enough children, Butz said.

The U.S. is the fastest growing industrialized nation in the world, adding about 2.8 million people a year. That's a little less than 1 percent, but enough to mitigate the kinds of problems facing Japan and many European countries.

Europe, with 728 million people, saw its population shrink by 74,000 since the beginning of the decade, according to the United Nations. By 2050, it is projected to lose a total of 75 million people.

That ought to give motorists on Germany's Autobahn some extra room to change lanes. But experts warn it could cause labor shortages while straining retirement and health programs, ultimately threatening economic competitiveness.

The problem is that birth rates are so low there aren't enough young people entering the work force to support an aging population, said Hans-Peter Kohler, an associate professor of sociology at the University of Pennsylvania.

"Presumably, many people would not be so concerned about the numbers declining if it wasn't combined with an aging population," Kohler said. "I think it's more the age structure that gives rise to these concerns, and these concerns are well justified."

Russian President Vladimir Putin is so concerned he recently proposed paying women money to have children. Last year, France increased monthly stipends to parents who take time off work to care for a third child.

When Japan announced in June that its population had shrunk in 2005 for the first time, Prime Minister Junichiro Koizumi said, "The data must be accepted gravely."

On Friday, Japan announced that it is now the world's most elderly nation, with more than a fifth of its people 65 or older. Italy is second.

On average, women must have 2.1 children in their lifetimes for a society to replenish itself, accounting for infant mortality and other factors. Only one country in Europe — Albania — has a fertility rate above 2, according to statistics gathered by the Central Intelligence Agency. Russia's fertility rate is 1.28. In Japan, it's 1.25.

"We're going to have the chance to learn from Europe," Butz said. "For better or worse, they are leading the world into something that has never happened before."

John Seager, president of Population Connection, predicted that any adverse affects of shrinking populations will be temporary.

"It may be the only good crisis we ever had," said Seager, whose group, formerly known as Zero Population Growth, advocates lower birth rates.

America is getting older, too — the oldest baby boomers turn 60 this year — and there have been consequences. Private pensions are failing at an alarming rate and Social Security, if left unchanged, is projected to drain the money in its trust fund by 2041.

Twelve percent of the U.S. population is 65 or older, a share that is projected to grow. But two factors keep America younger than Europe: higher fertility rates and immigration.

The United States has a fertility rate of 2.05, about enough to maintain a stable population. The U.S. also adds people through immigration, something many European countries have shunned.

About 40 percent of U.S. population growth comes from immigration, both legal and illegal, according to the Census Bureau. However, if the fertility rate remains unchanged, all of America's population increase will eventually come from immigration, Butz said.

The immigration issue has preoccupied Washington and much of the country for the past year, with Congress working on legislation that would tighten borders and, perhaps, create an avenue to citizenship for many of the 11 million to 12 million illegal immigrants.

Advocates argue that immigrants take jobs that would go unfilled by people born in this country. They note that the number of immigrants in the U.S. has nearly quadrupled in the past 40 years, to about 36 million, and unemployment remains low.

Opponents accuse immigrants of driving down wages and adding to an increasingly crowded country.

"Population growth kind of cuts both ways," said Steven Camarota, research director for the Center for Immigration Studies, which advocates tighter restrictions on immigration.

"If you are someone who sells cars for a living, you've got more people to sell cars to," he said. "But if you are someone who drives, you have a lot more cars to contend with."

A USA Today/Gallup Poll found that 39 percent of adults think U.S. population growth is a major problem, and 57 percent think it will be a major problem in the future.

But even with immigration, the nation's growth rate is slowing. The number of people had been doubling about every half century, from 75 million in 1900 to 150 million in 1950. The Census Bureau projects it will hit 300 million sometime in October.

By 2050, the United States is projected to have about 420 million people.

Many developing countries, meanwhile, are facing population explosions. India is projected to add 473 million people by 2050 for a new total of 1.6 billion, sending it past China as the world's most populous country.

Some smaller nations will grow significantly as well, with Uganda projected to more than quadruple its population and Niger projected to more than triple its size.

At the other end is Ukraine, which is projected to lose more than 40 percent of its population, shrinking to 26 million.

China, with its controversial one-child policy, is expected to maintain a stable population for the next four decades, leaving it the second most populous country. The United States would remain third.

"Most people would consider moderate population growth preferable to the alternatives," said Kohler, the sociology professor. "I would say that's where the U.S. falls."