The report, "Fertility of American Women: 2006" also found that of the 4.2 million women who had given birth in the previous 12 months, 36 percent were separated, widowed, divorced or never married when the survey was conducted, the Census Bureau said in a release. Of the 1.5 million unmarried mothers, 190,000 were living with a partner.

The highest levels of fertility -- 67 births per 1,000 women in the year before to the survey -- were among those with a graduate or professional degree, the bureau said. Second-generation Hispanic women tended to have lower fertility rates than either third-generation or foreign-born Hispanics.

The fertility report, issued Monday, is the first Census Bureau study on this topic to use data from the American Community Survey, allowing for state-by-state comparisons of fertility characteristics, the bureau said. Information from the Current Population Survey also was included to provide historical comparisons on levels of childlessness and births.