More babies were born in the United States in 2007 than in any other year in American history, according to preliminary data reported Wednesday by the National Center for Health Statistics.

The 4,317,000 births in 2007 just edged out the figure for 1957, at the height of the baby boom. The increase reflected a slight rise in childbearing by women of all ages, including those in their 30s and 40s, and a record share of births to unmarried women.

But in contrast with the culturally transforming postwar boom, when a smaller population of women bore an average of three or four children, the recent increase mainly reflects a larger population of women of childbearing age, said Stephanie J. Ventura, chief of reproductive statistics at the center and an author of the new report. Today, the average woman has 2.1 children.

Also in 2007, for the second straight year and in a trend health officials find worrisome, the rate of births to teenagers rose slightly after declining by one-third from 1991 to 2005.