More babies were born in the United States in 2007 than ever before, driven by increases in fertility rates, teen birth rates and childbearing by unmarried women, according to new government data released Wednesday.

The preliminary report from the National Center for Health Statistics is gleaned from birth records and shows that most of the growth - to a record 4.31 million births - was fueled by adult women. The previous record for births in a single year was in 1957 with 4.30 million. Birth rates rose for women in their 20s, 30s and early 40s. An estimated 1.7 million babies were born to unmarried women, comprising just under 40 percent of all births in the U.S. Teens accounted for 23 percent of births.

But even though 2007 recorded the highest number of births in the U.S. for a single year, demographers aren't crying "baby boom."

"I don't think the U.S. was heading to a baby boom that would have mirrored the 1950s," says Hans-Peter Kohler, a research associate at the University of Pennsylvania's Population Studies Center.

Stephanie Ventura, a demographer who worked on the government report, says birth numbers are higher due to a larger population overall rather than individual women having multiple children as was the case during the boom.

But Kohler says it's "striking" that the USA has had a sustained increase in birth rates at a time when such rates are declining in many developed countries. And he says it's remarkable that the U.S. increases occur "across so many different population groups and so many age categories."

The 2007 data found:

- A second straight year of rising teen birth rates: Rates inched up 1 percent to 42.5 births per 1,000 females ages 15-19 from 41.9 births per 1,000 in 2006. That year noted a 3 percent hike.

- The highest U.S. fertility rate since 1990: The rate for women ages 15-44 rose by 1 percent to 69.5 births per 1,000 women in 2007.

- Increases in unmarried childbearing, termed at "historic levels" in the report: The total number of births, birth rate, and proportion of births to unmarried women all increased 3 percent to 5 percent.

Because the data is pre-recession, demographers say 2009 data will be the first to really reflect the effect of the economic crisis on fertility.

"The increases we've seen in the past few years will certainly stop and I would really expect to see a little bit of a decline," says Carl Haub, senior demographer at the non-profit Population Reference Bureau based in Washington, D.C.

Evidence from the Depression and past recessions has shown that numbers of births drop during hard economic times.

However, University of Chicago economist and sociologist Gary Becker says that dictum may not hold true anymore due to greater numbers of women in the labor force. He suggests that women laid off from their jobs might see unemployment as the time to have a child, saying "births might go up during recession."

As for the teen birth rate, John Santelli, an adolescent medicine specialist at Columbia University, says the 2006 and 2007 increases are significant because individual states had been reporting changes as far back as 2003 and 2004.

"It's a true reversal at this point," he says. "The increase in the birth rate lines up very well with changes in teen sexual behaviors that lead to pregnancy."

In the 1990s when teen birth rates began declining, Santelli says "fewer kids were having sex, having fewer partners and there was an enormous increase in condom use."