What were they thinking, exactly, isn't clear. The Centers for Disease Control and Prevention survey didn't ask teens to explain their reasoning.
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.
This summer, a national statistical report showed a drop in teen pregnancy rates for the second year in a row, to a rate of 20.1 babies per 1,000 girls ages 15 to 17.
Image: Pregnant woman's belly, via Shutterstock