More babies were born in the United States last year than ever before, according to preliminary data, but it's not another baby boom just yet.

About 4,315,000 children were born in 2007, about 15,000 more births than the peak time of the baby boom in 1957, said Stephanie Ventura, a demographer at the National Center for Health Statistics, which compiled the data from provisional birth certificate registrations at state health departments.

"According to our provisional data, we had the highest number of births ever reported in 2007," she said.

But this doesn't mean there's a baby boom under way. In the mid-'50s about as many babies were being born from a lot fewer women.

"What this really reflects is that the population has grown so much in the United States, so you naturally expect more births," Ventura said. "It's hard to call this a baby boom at this point."

Years leading up to 2007 showed slight increases in the number of births, not the substantial growth of the baby boom years, which were 1946 through 1964, Ventura said.