"We're continuing to see strong growth in the South and the West," said Paul Mackun, a geographer with the U.S. Census Bureau. This growth bolsters a country that, despite a comparatively low fertility rate, is the third most populous country in the world and gains about 2.5 million residents a year.

Las Vegas, West Lead U.S. Growth

As it has for the past half-century, the West leads the United States in population growth. The West, which grew 19.7 percent in the last ten years, and the South (with 17.3 percent growth) were both above the country's average population growth of 13.2 percent.

Growth in the western United States was particularly bolstered by Nevada, which grew by 66 percent in the past ten years—the stongest U.S. state growth. Like other regions, the bulk of Nevada's growth came in urban and suburban areas. The Las Vegas, Nevada/Arizona metropolitan area grew at the fastest rate in the country—more than 83 percent.

Arizona, Colorado, Utah, and Idaho rounded out the top five state population jumps in the country. Colorado's Douglas County, located south of Denver, grew nearly 200 percent in the last ten years—the greatest county growth in the United States. Four additional counties—located in areas surrounding Denver and Atlanta, Georgia—saw their populations more than double since 1990.

Midwest, Northeast See Low Growth

Although northeastern and midwestern metropolitan areas remain some of the most highly populated in the United States (New York, Chicago, Washington, Philadelphia, Boston, and Detroit are in the top ten), these regions saw the country's lowest growth.

The Midwest and Northeast each saw population growth between 1990 and 2000 (7.9 and 5.5 percent, respectively), but remained below the national average of 13.2 percent.

Decline in the Northeast and Midwest was due in part to the loss of residents in the "industrial belt," said Mackun.

While no state as a whole declined in population, the population of several of the regions' smaller cities declined, including Hartford, Connecticut; St. Louis, Missouri; Gary, Indiana; Baltimore, Maryland; and Flint, Michigan—each of which lost more than ten percent of its population.

A stretch of counties from North Dakota south to Texas also saw a decrease in population—some more than 20 percent in the last ten years. "These counties are primarily sparsely populated, non-metropolitan counties," noted Mackun.

The U.S. Census Bureau has predicted three possible future growth patterns for the United States. The highest prediction, allowing for an increase in the country's current fertility and immigration rates, indicates that the United States population may reach as high as half a billion by 2050.