A new government study suggests a lot of teenage girls are clueless about their chances of getting pregnant.

In a survey of thousands of teenage mothers who had unintended pregnancies, about a third said they didn't use birth control because they didn't believe they could pregnant.

Other studies have asked teens about their contraception use and beliefs about pregnancy. But the new report is the first to focus on teens who didn't intend to get pregnant but did.

The Centers for Disease Control and Prevention released the study on Thursday.

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Online:

CDC report: http://www.cdc.gov/mmwr