In a survey of thousands of teen mothers who had unintended pregnancies, about a third who didn't use birth control said the reason was they didn't believe they could get pregnant.
Why they thought that remains unclear. The Centers for Disease Control and Prevention didn't ask the teens to explain.
Other researchers who talked to teen mothers say they believed they couldn't get pregnant the first time they had sex, didn't think they could get pregnant during their periods or thought they were sterile.
While other studies have asked teens about their contraception use and beliefs about pregnancy, the CDC report is the first to focus on teens who didn't want to get pregnant but did.
The researchers interviewed about 5,000 teenage girls in 19 states who gave birth after unplanned pregnancies from 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 percent said they didn't use contraception the last time they had sex.
When asked what kind of birth control they used, nearly 20 percent of the teen moms said they used the pill or a birth control patch. Another 24 percent said they used condoms.
CDC officials said they do not think that the pill, condoms and other forms of birth control were faulty. Rather, they think the teens failed to use it correctly or consistently.
About 13 percent of those not using contraception said they didn't because they had trouble getting it. Also, nearly a quarter of the teen moms who did not use contraception said they didn't because their partner did not want them to.
The CDC report states approximately 400,000 teens between the ages 15 and 19 give birth every year in the U.S., and the teen birth rate remains the highest in the developed world.