Births fell 2.7 percent last year even as the population grew, numbers released Friday by the National Center for Health Statistics show.
"It's a good-sized decline for one year. Every month is showing a decline from the year before," said Stephanie Ventura, the demographer who oversaw the report.
The birth rate, which takes into account changes in the population, fell to 13.5 births for every 1,000 people last year. That's down from 14.3 in 2007 and way down from 30 in 1909, when it was common for people to have big families.
"It doesn't matter how you look at it fertility has declined," Ventura said.
The situation is a striking turnabout from 2007, when more babies were born in the United States than any other year in the nation's history. The recession began that fall, dragging stocks, jobs and births down.
"When the economy is bad and people are uncomfortable about their financial future, they tend to postpone having children. We saw that in the Great Depression the 1930s and we're seeing that in the Great Recession today," said Andrew Cherlin, a sociology professor at Johns Hopkins University.
"It could take a few years to turn this around," he added, noting that the birth rate stayed low throughout the 1930s.
Another possible factor in the drop: a decline in immigration to the United States.
The downward trend invites worrisome comparisons to Japan and its lost decade of choked growth in the 1990s and very low birth rates. Births in Japan fell 2 percent in 2009 after a slight rise in 2008, its government has said.
Not so in Britain, where the population took its biggest jump in almost half a century last year and the fertility rate is at its highest level since 1973. France's birth rate also has been rising; Germany's birth rate is lower but rising as well.
"Our birth rate is still higher than the birth rate in many wealthy countries and we also have many immigrants entering the country. So we do not need to be worried yet about a birth dearth" that would crimp the nation's ability to take care of its growing elderly population, Cherlin said.
The new U.S. report is a rough count of births from states. It estimates there were 4,136,000 births in 2009, down from 4,251,095 in 2008 and more than 4.3 million in 2007.
The report does not give details on trends in different age groups. That will come next spring and will give a clearer picture who is and is not having children, Ventura said.
Last spring's report, on births in 2008, showed an overall drop but a surprising rise in births to women over 40, who may have felt they were running out of time to have children and didn't want to delay despite the bad economy.
Women postponing having children because of careers also may find they have trouble conceiving, said Mark Mather of the Population Reference Bureau, a Washington-based demographic research group.
"For some of those women, they're going to find themselves in their mid-40s where it's going to be hard to have the number of children they want," he said.
Heather Atherton is nearing that mark. The 35-year-old mom started a home-based public relations business after having a baby girl in 2003. She and her husband upgraded to a larger home in 2005 and planned on having a second child not long afterward. Then the recession hit, drying up her husband's sales commissions and leaving them owing more on their home than it is worth. A second child seemed too risky financially.
"However, we just recently decided that it's time to stop waiting and just go for it early next year and let the chips fall where they may," she said. "We can't allow the recession to dictate the size of our family. We just need to move forward with our lives."