More women than ever are choosing to ignore their "biological clocks."

A Census Bureau report released this week shows that women are waiting longer or choosing not to have children at all these days. The "Fertility of American Women" report, based on 2006 numbers, shows that 20 percent of women 40 to 44 were childless in 2006, twice as high as the level 30 years earlier.

And those moms in that age bracket have an average of 1.9 children each, more than one child fewer than women of the same age in 1976; the median number of children in 1976 was 3.1.

"A lot of women are not having any children," Jane Lawler Dye, a Census Bureau researcher who did the report, told The New York Times. "It used to be sort of expected that there was a phase of life where you had children, and a lot of women aren’t doing that now."

The study also found that in 2006:

• 57% of women with a recent birth were in the labor force.

• Of the 4.2 million women who had a birth in the previous 12 months, 36% were separated, widowed, divorced or never married at the time of the survey. Of these 1.5 million unmarried moms, 190,000 were living with an unmarried partner.

• Second-generation Hispanic women tend to have lower fertility rates than either foreign-born Hispanics or those who were third generation.

• The highest levels of current fertility (67 births in the year prior to the survey per 1,000 women) were among those with a graduate or professional degree.

• Mothers in the District of Columbia, Mississippi and North Carolina were the most likely to have never married. New moms in California were the most likely to be foreign-born, while those in Mississippi were the likeliest to be poor.

• The national birth rate for women age 15 to 50 on welfare or other forms of public assistance was about three times of those not receiving government help (the report cites welfare reform passed in 1996 as perhaps contributing to this).

"Clearly women have competing alternatives for the use of their time, with the labor market and employment being one and delayed marriage, which has been another trend,” Suzanne Bianchi, chairwoman of the sociology department at the University of Maryland, told the Times. “The interesting question is, has it stopped? Is this it, or will we see even higher rates of childlessness among future generations?"