Approximately 400,000 teens aged 15-19 years give birth every year in the United States (1), and the teen birth rate remains the highest in the developed world. Teen childbearing is a public health concern because teen mothers are more likely to experience negative social outcomes, including school dropout. In addition, infants of teen mothers are more likely to be low birth weight and have lower academic achievement, and daughters of teen mothers are more likely to become teen mothers themselves. To learn why teens wishing to avoid pregnancy become pregnant, CDC analyzed data from the 2004-2008 Pregnancy Risk Assessment Monitoring System (PRAMS). This report describes estimated rates of self-reported prepregnancy contraceptive use among white, black, and Hispanic teen females aged 15-19 years with unintended pregnancies resulting in live births. Approximately one half (50.1%) of these teens were not using any method of birth control when they got pregnant, and of these, nearly one third (31.4%) believed they could not get pregnant at the time; 21.0% used a highly effective contraceptive method (although less than 1% used one of the most effective methods, such as an intrauterine device [IUD]); 24.2% used the moderately effective method of condoms; and 5.1% used the least effective methods, such as rhythm and withdrawal. To decrease teen birth rates, efforts are needed to reduce or delay the onset of sexual activity, provide factual information about the conditions under which pregnancy can occur, increase teens' motivation and negotiation skills for pregnancy prevention, improve access to contraceptives, and encourage use of more effective contraceptive methods.