The report may help explain why the teen pregnancy rate is no longer dropping like it was.
But there were some notable exceptions in the new survey by the Centers for Disease Control and Prevention. About 17 percent of sexually experienced teen girls say they had used the rhythm method—timing their sex to avoid fertile days to prevent getting pregnant. That's up from 11 percent in 2002.
They may have been using another form of birth control at the same time. But the increase is considered worrisome because the rhythm method doesn't work about 25 percent of the time, said Joyce Abma, the report's lead author. She's a social scientist at the CDC's National Center for Health Statistics.
The survey results were based on face-to-face interviews with nearly 2,800 teens ages 15 through 19 at their homes in the years 2006 through 2008. Trained female interviewers asked the questions.
It found that about 42 percent of never-married teens had had sex at least once in their life. Of those teens, 98 percent said they'd used birth control at least once, with condoms being the most common choice. Those findings were about the same as in the 2002 survey.
The increase in the rhythm method may be part of the explanation for recent trends in the teen birth rate. The teen birth rate declined steadily from 1991 through 2005, but rose from 2005 to 2007. It dropped again in 2008, by 2 percent, to about 10 percent of all births.
"We've known the decline in childbearing stalled out. This report kind of fills in the why," said Bill Albert, a spokesman for the National Campaign to Prevent Teen Pregnancy.
Teen attitudes may be big part of it. Nearly 64 percent of teen boys said it's OK for an unmarried female to have a child, up from 50 percent in 2002. More than 70 percent of teen girls agreed, up from 65 percent, though the female increase was not statistically significant.
The survey was conducted at a time of some highly publicized pregnancies of unmarried teens, including Bristol Palin, the daughter of former Republican vice presidential candidate Sarah Palin, and Jamie Lynn Spears, Britney's kid sister. The 2007 movie "Juno," a happy-ending tale of a teen girl's accidental pregnancy, was popular at the time.