Washington - The U.S. birth rate plunged last year to a record low, with the decline being led by immigrant women hit hard by the recession, according to a study released Thursday by the Pew Research Center.

The overall birth rate declined by 8 percent between 2007 and 2010, with a decrease of 6 percent among U.S.-born women and 14 percent among foreign-born women. The decline for Mexican immigrant women was more extreme, at 23 percent. The overall birth rate is now at its lowest since 1920, the earliest year with reliable records.

The decline could have far-reaching implications for U.S. economic and social policy. A continuing decline would challenge long-held assumptions that births to immigrants will help maintain the U.S. population and provide the taxpaying workforce needed to support the aging baby boomer generation.

The U.S. birth rate - 63.2 births per 1,000 women of child-bearing age - has fallen to just over half of what it was at its peak in 1957. The rate among foreign-born women also had been declining in recent decades, according to the report, though more slowly.

But after 2007, as the worst recession in decades dried up jobs and economic prospects across the nation, the birth rate for immigrant women abruptly plunged.

The fall is not because there are fewer immigrant women of childbearing age, but because of a change in their behavior, said D'Vera Cohn, an author of the report, adding that "the economic downturn seems to play a pretty large role in the drop in the fertility rate."

While the declining U.S. birth rate has not yet created the stark imbalances in graying countries such as Japan or Italy, it should serve as a wake-up call for policymakers, said Roberto Suro, a professor of public policy at the University of Southern California.

"We've been assuming that when the baby boomer population gets most expensive, that there are going to be immigrants and their children who are going to be paying into (programs for the elderly), but in the wake of what's happened in the last five years, we have to re-examine those assumptions," he said. "When you think of things like the solvency of Social Security, for example . . . relatively small increases in the dependency ratio can have a huge effect."

The falling birth rate mirrors what has happened during other recessions. A Pew study last year found that a decline in U.S. fertility rates was closely linked to hard times, particularly among Hispanics.

"The economy can have an impact on these long-term trends, and even the immigrants that we have been counting on to boost our population growth can dip in a poor economy," said William H. Frey, a demographer at the Brookings Institution, noting that Hispanic women, who led the decline, occupy one of the country's most economically vulnerable groups.

Historically, once the economy rebounds after a recession, so does the birth rate, Cohn said.

But other factors may also be affecting the decline, and may not change much once the economy recovers.

A vast portion - 47 percent - of immigrants to the U.S. are of Hispanic origin. But in recent years immigration from Mexico, the biggest contributing country, has dried up; for the first time since the Great Depression, the net migration from Mexico has been zero.

Latino immigrants who have been here longer tend to adopt U.S. attitudes and behavior, including having smaller families, Suro said. He added that the sharp decline in the birth rate among Mexican immigrants may be explained by the fact that the rate was so high that there was more room for it to fall.

And while the Hispanic birth rate may never return to its highest levels, immigrants who have babies will likely continue to boost overall fertility rates, said Frey, who saw the current decline as a "short-term blip." Immigrants from Asia, he said, continue to move to the United States, though their birth rates are not likely to approach that of Hispanic immigrants at their peak.

The recent birth rate decline among Latino women may also be related to enhanced access to emergency contraception and better sex education in recent years, said Kimberly Inez McGuire, a senior policy analyst at the National Latina Institute for Reproductive Health, based in New York.