by Leigh Ann Woodruff, May 20, 2012

Declining fertility rates are one of the factors driving a decline in U.S. population growth, according to a Population Reference Bureau (PRB) analysis of U.S. Census Bureau data. Lower immigration levels and population aging are additional factors in declining population growth.

According to the analysis, from 2010 to 2011, the U.S. population grew 0.7 percent after averaging 0.9 percent each year from 2000 to 2010.

"There's two parts of this," says Mark Mather, PRB associate vice president for Domestic Programs. "There's the declining fertility rate, which is the number of births per woman, and there's the declining total number of births, which is a different issue and sometimes caused by other factors, including trends in immigration. If there's fewer people living in the United States who are of reproductive age, then that can also contribute to a decline in the number of births just because there's fewer potential parents."