But that doesn't mean women are waiting for the right moment: The study also found that half of mothers surveyed said parenthood "just happened."
While most women giving birth are doing it within the context of marriage, researchers said a record 41 percent of births were to unmarried women in 2008. That's up from 28 percent in 1990, according to the study, "The New Demography of American Motherhood." The trend crossed major racial and ethnic groups.
Nearly 14 percent of mothers of newborns were 35 or older two years ago -- and only about 10 percent were in their teens. The age trend was reversed in 1990, when teens had a 13 percent share of births.
"I think everyone will welcome a decline in births to teens," said D'Vera Cohn, a senior writer on the study. "It's notable that the population of teens is larger than it used to be, so there were more who could have become teen mothers."
Today, one in seven babies is born to a mother at least 35 years old. In 1990, one in 11 had a mother in that age group.
Most mothers of newborns (54 percent) had at least some college education in 2008, an increase from 41 percent in 1990. Among mothers 35 or older, 71 percent had at least some college education.
Improvements in medical care and fertility treatment, along with marriage and childbearing postponed to seek additional education, all factor into the shifts.
"The rise in women's education levels has changed the profile of the typical mother of a newborn baby," the report said. Cohn added that a lower share of mothers ended their education after high school, "so some of those mothers who would have been high school graduates in 1990 have some college education today."
The report is based on data from the National Center for Health Statistics and the U.S. Census Bureau, along with a telephone survey in April 2009 of about 1,000 parents, likely parents and other adults of both genders.
Overall, there were 4.3 million births in the U.S. in 2008, compared with 4.2 million in 1990. The number had risen each year from 2003 to 2007, then dipped in an apparent link to the economic downturn, the researchers said.
When American parents are asked why they decided to have a child, most cite "The joy of having children," the study said. For nearly half of parents, though, an important explanation is: "It wasn't a decision; it just happened."
Women surveyed were more likely than men to cite "it just happened" as somewhat or very important in their decision to give birth the first time.
Stephanie Coontz, director of research and public education for the nonprofit Council on Contemporary Families and a writer who teaches history and family studies at The Evergreen State College in Olympia, Wash., said the rise of single motherhood is significant.
"It's yet another nail in the coffin in the hope that we can solve the challenges facing us today by shoehorning everyone back into marriages," she said. "One of the big problems with that at this point is very often kids do worse if their mother rushes into a marriage that may be unstable."
Multiple births associated with the trend toward older motherhood were up sharply, including a 70 percent increase in the twin birth rate from 1980 to 2004.
"Not only are women in their 30s more likely than younger women to conceive multiples on their own, they also are more likely to undergo fertility treatments, which are linked to births of multiples," the researchers said.