While out-of-wedlock births have long been associated with teen mothers, the teen birth rate actually dropped last year to the lowest level on record. Instead, births among unwed mothers rose most dramatically among women in their 20s.
Experts said the overall rise reflects the burgeoning number of people who are putting off marriage or living together without getting married. They said it also reflects the fact that having a child while unmarried is more acceptable than in the past.
The increase in births to unwed mothers was seen in all racial groups, but rose most sharply among Hispanics. It was up among all age groups except youngsters ages 10 to 17.
"A lot of people think of teenagers and unmarried mothers synonymously, but they are not driving this," said Stephanie Ventura of the National Center for Health Statistics, a co-author of the report.
The government also reported that the rate of births by Caesarean delivery continued to climb in 2005 to a record high, despite efforts by public health authorities to bring down the number.
Many experts believe a large number of C-sections are medically unnecessary and done only for the convenience of the mother or her doctor.
C-Sections are up by almost 50 percent in the past decade and up 3 percent last year, CBS News correspondent Dan Raviv reports.
The government report includes information from 99 percent of U.S. birth certificates filed last year. The information for 2005 is considered preliminary, but officials said it is not expected to change much.
About 4.1 million babies were born in the United States last year, up slightly from 2004. More than 1.5 million of those were to unmarried women; that is about 37 percent of the total. In 2004, about 36 percent of births were out of wedlock.
Births by unmarried mothers have been rising since the late 1990s.
Several factors may be contributing to the trend, said Dr. Yolanda Wimberly, an adolescent-medicine specialist at Atlanta's Morehouse School of Medicine.
More women in their 30s and 40s, hearing their biological clock, are choosing to give birth despite their single status. Younger women are not as worried about being unmarried, either, she added.
"I think it's more acceptable in society" to have a child without getting married, she said.
Just because a mother is not married does not mean the father isn't around, Ventura noted. She cited 2002 statistics that showed that about 20 percent of all new mothers under 20 were unmarried but living with the father at the time of the birth. That same was true of about 13 percent of all new mothers ages 20 to 24.
According to census figures, the median age at first marriage was 27 for men and 25 for women last year, up from 23 and 20 in 1950. Meanwhile, the number of unmarried-couple households with children has been climbing, hitting more than 1.7 million last year, up from under 200,000 in 1970.
Other findings in the report: