"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.