Women are waiting longer to have children, and more women than ever are choosing not to have children at all, according to a new Census Bureau report.

Twenty percent of women ages 40 to 44 have no children, double the level of 30 years ago, the report said; and women in that age bracket who do have children have fewer than ever  an average of 1.9 children, compared with the mean average of 3.1 children in 1976.

“A lot of women are not having any children,” said Jane Lawler Dye, a Census Bureau researcher who did the report, which looked at women of childbearing age in 2006. “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,” Ms. Dye said.

Women with advanced degrees are more likely to be childless, the study found. Of women 40 to 44 with graduate or professional degrees, 27 percent are childless, compared with 18 percent of women who did not continue their education beyond high school, the data show.