In a survey of thousands of teenage mothers who had unintended pregnancies, about a third who didn't use birth control said the reason was they didn't believe they could pregnant.
Why they thought that isn't clear. The Centers for Disease Control and Prevention survey didn't ask teens to explain.
But other researchers have talked to teen moms who believed they couldn't get pregnant the first time they had sex, didn't think they could get pregnant at that time of the month or thought they were sterile.
"This report underscores how much misperception, ambivalence and magical thinking put teens at risk for unintended pregnancy," said Bill Albert, a spokesman for the Washington, D.C.-based National Campaign to Prevent Teen and Unplanned Pregnancy.
Other studies have asked teens about their contraception use and beliefs about pregnancy. But the CDC report released Thursday is the first to focus on teens who didn't want to get pregnant but did.
The researchers interviewed nearly 5,000 teenage girls in 19 states who gave birth after unplanned pregnancies in 2004 through 2008. The survey was done through mailed questionnaires with telephone follow-up.
About half of the girls in the survey said they were not using any birth control when they got pregnant. That's higher than surveys of teens in general, which have found that fewer than 20 per cent said they didn't use contraception the last time they had sex.
"I think what surprised us was the extent to which they were not using contraception," said Lorrie Gavin, a CDC senior scientist who co-authored the report.
Some of the teen moms were asked what kind of birth control they used: Nearly 20 per cent said they used the pill or a birth control patch. Another 24 per cent said they used condoms.
CDC officials said they do not believe that the pill, condoms and other forms of birth control were faulty. Instead, they think the teens failed to use it correctly or consistently.
Only 13 per cent of those not using contraception said they didn't because they had trouble getting it.
Another finding: Nearly a quarter of the teen moms who did not use contraception said they didn't because their partner did not want them to. That suggests that sex education must include not only information about anatomy and birth control, but also about how to deal with situations in which a girl feels pressured to do something she doesn't want to, Albert said.
The findings are sobering, he added. But it's important to remember that the overall teen birth rate has been falling for some time, and recently hit its lowest mark in about 70 years.
Albert said it would be a mistake to come away from the report saying, "They can't figure this out?" ''Most of them are figuring it out," he said.