The U.S. birthrate hit a record low last year, says a new analysis by the Pew Research Center. The declining birthrate is mostly associated with immigrants having fewer babies now than earlier.

Birth-rate in the country declined by 8 percent from 2007 to 2010; for women born in the U.S., the birth rate decreased 6 percent, but for women who have migrated to the U.S., the birth rate dropped 14 percent.

About 4.0 million babies were born in the U.S. in 2010; about 3.1 million of them to U.S. born women and 930,000 to immigrant women. Last year (2011), 3.95 million babies were born in the country. According to data from the National Center for Health Statistics, the birth rate in 2011 was an estimated 63.2 percent for 1,000 women of childbearing age; a rate that is the lowest since 1920s.

Analysts say that the decrease in the number of babies born to immigrant women is mostly due to behavioral changes and not due to change in the number of women of childbearing age. Immigrants continue to produce more children disproportionate share of the nation's newborns, the report said.

The U.S. hasn't reached a point where there is a huge imbalance between the old and the young people in the country like Japan or Italy. However, with the recent drop in birth rates, there could be a shortage of young people.

"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 reexamine those assumptions. 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," said Roberto Suro, a professor of public policy at the University of Southern California, reports The Washington Times.

Population projection by Pew Research Center shows that people coming into the country will continue to play a larger role in the U.S. population growth. More than 80 percent of population growth in the U.S., by 2050 will be due to immigrants who have come in the country since 2005.