About 37 percent of births in the United States are the result of unintended pregnancies, a proportion that has remained fairly steady since 1982, according to new research from the National Center for Health Statistics, a branch of the U.S. Centers for Disease Control and Prevention.
The numbers are not surprising to many doctors and researchers, and many said it's discouraging that they have not improved in three decades.
"Trying to prevent unintended births is sort of an increasingly difficult task," said William Mosher, a statistician at NCHS and the study's lead author.
Researchers interviewed more than 12,000 women from 2006 to 2010 who had given birth to live babies. Their findings showed changes in who is giving birth in the United States, planned or not: In 1982, white, married women accounted for 66 percent of births in the United States; today, that group accounts for 43 percent of total births.
But the findings also portrayed sharp demographic contrasts in women who have untended pregnancies:
About 23 percent of married women had an unintended pregnancy, compared with 50 percent of unmarried women who were living with their baby's father and 67 percent of unmarried women not living with the baby's father.
Nearly 77 percent of teens' pregnancies were unintended, compared with 50 percent of women ages 20 to 24 and 25 percent of women ages 25 to 44.
Almost 17 percent of women with a college degree had unintended pregnancies, compared with 41 percent of women without a high school diploma.
Nearly 54 percent of black women reported an unintended pregnancy, compared with 43 percent of Hispanic women and about 31 percent of white women.
"These are staggering statistics," said Sheryl Kingsberg, a professor of reproductive biology and psychiatry at University Hospitals Case Medical Center in Cleveland, who was not involved in the study. "Here we are with various means of effective birth control at our fingertips, but it's not reaching the population that needs it the most."
Previous studies have found that about half of unintended births come from ineffective use of contraception -- not wearing a condom or inappropriately taking birth control pills, for example. Others simply don't use contraception at all.
Some doctors say a lack of education about and access to contraception through the health care system are the prohibiting factors driving those behaviors for many women, especially teens and women with lower incomes and education.
In the current study, more than one-third of women who had unintended births reported that they didn't think they could get pregnant.
Mosher said that points to a serious sex education problem among American women.
"Basically what that suggests is that many women think that because they have not used a method and have not gotten pregnant in two or three or four acts of intercourse that they're sterile. And of course, that's not how it works," he said.
The rates of unintended pregnancies have persisted even as new, more advanced contraceptive methods have been developed -- things like intrauterine devices, vaginal rings and implants that don't require remembering to take a pill every day. But those methods are more expensive than other types of birth control, and many women simply may not be aware that they exist.