New mothers in the U.S. are increasingly older and better educated than they were two decades ago, according to a study on the state of American motherhood released Thursday by the Pew Research Center.

But that doesn’t mean women are waiting for just the right moment: The study also found that half of mothers surveyed said that parenthood “just happened.”

While most women giving birth are doing it within the context of marriage, the study found 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 2006, 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 for 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.