More than one-third of births in the U.S. are the result of an unplanned pregnancy, according to a CDC study.

Despite a decrease in unplanned pregnancies among white women, researchers say the number of unintended births has remained steady at about 37% of all births since 1982.

"The growing proportion of births to unmarried women, most of which were unintended, has kept the overall proportion of unintended births approximately constant," write researcher William Mosher, PhD, of the National Center for Health Statistics at the CDC, and colleagues.

The study is published in the National Health Statistics Reports. It shows that unmarried women accounted for 41% of all births in 2009, up from 18% in 1980.

Researchers say major differences also persist in the number of unplanned pregnancies according to:

    age
    race
    income
    education

"Unintended births occur disproportionately among non-Hispanic black women, unmarried women, and women with less income and education," they write. "For example, the proportion of all births that were unwanted or mistimed by two years or more was 7% for college graduates compared with 35% for women who did not complete high school."

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Unplanned Pregnancy Statistics

The study shows that never-married women and Hispanic women now account for a growing number of births.

Researchers say the percentage of births that were intended among white women who had ever been married has increased during recent years. But these women now represent a much smaller share of overall births. In 1981, they accounted for 66% of all births. But in 2006-2010, this number decreased to 43%.

The study also showed that 60% of women who experienced an unplanned pregnancy in 1998-2002 were not using contraception.

More than a third of those who did not use contraception (36%) said they did not think they could get pregnant.

"Underestimating the risk of pregnancy is the most common reason for not using contraception that leads to unintended pregnancy," the researchers write. "There was no significant variation in the percentage of women who gave this reason by age, marital status, or income. However, Hispanic women were more likely than others to say they did not think they could get pregnant (49%, compared with 35% of white women and 25% of black women)."

Researchers say the most recent unplanned pregnancy statistics from 2006-2010 highlight persistent differences according to the mother's age, ethnicity, and marital status.

During this period:

    Nearly 4 out of 5 teen births were the result of an unplanned pregnancy.
    Half of births to women aged 20-24 were intended, compared with 75% of births among women aged 25-44.
    About 77% of births to women who were married at the time of birth were intended, compared with 49% of births to women who were living with their partner and 33% of births among single women.
    More than two-thirds of births to white women were intended, compared with 57% among Hispanic women and 47% among African-American women.