The population of the U.S. is projected by analysts to reach 300 million this morning, less than 40 years after the 200 million milestone, according to the U.S. Census Bureau.

The U.S. population reached 100 million in 1915 and 200 million in 1967, doubling in a span of 52 years.

According to the Census Bureau's current projections, the U.S. will reach the 400 million mark in 2043, said Mark Tolbert, spokesman for the U.S. Census Bureau.

"We are, in fact, the third most populous country now behind China and India," Tolbert said.

The projection, which is an estimated number, is based on birth and death rates, as well as net immigration numbers.

Tolbert said the estimate is for current U.S. residents only. It does not include American citizens living in a foreign country, such as members of the U.S. armed forces.

David Eggebeen, a Penn State professor of human development and family studies, sociology and demography, said there are both sociological and economic factors affecting the size of the American family and the population of the United States.

"There's delaying in having that first child," he said, adding that Americans are also getting married later.

In the early 1950s, American couples were having an average of 3.5 children, while today the average is 1.9 children, just below replacement fertility rate, he said.

The reason many industrial countries experience population growth is because of immigration, Eggebeen said.

"Immigrant groups have higher fertility rates," he said.

It may be that immigration is not only responsible for America's increasing population but economic success as well, said Barry Ickes, a Penn State economics professor.

"More resources are good. More human resources are good too," Ickes said. The U.S. needs a growing working population to replace its labor force and support its Social Security system, which otherwise would have to be altered by raising taxes or cutting benefits, he said.

Ickes said a country with high per capita income like the U.S. is an ideal destination for immigrants. America is more lenient than other high-income countries with regard to immigration, so it tends to attract more immigrants, he said.

"Most of the European countries have almost below replacement fertility," Gordon De Jong, professor of sociology and demography, said. He said the U.S. has a lot of immigrants, but the key factor is that many are young and in their prime working years.

Whether the 300 million mark will put stress on America's health and social services is uncertain.

"This is a continuing debate," De Jong said. "The key is not just numbers."

De Jong explained that a number of factors go into whether a large population squeezes a country's resources. He said the age demographics of a country may play a bigger part in putting stress on the country's social services than population in general.

De Jong said one of the factors that allows the U.S. to support a large population with a high standard of living is its healthcare and education.

He added smaller countries sometimes have trouble supporting large populations because they don't have the same food production and distribution networks that large countries, like the U.S., have.