BACKGROUND Each year, nearly one in four U.S. women at risk of unintended pregnancy experience one or more months of contraceptive nonuse. Understanding what factors are associated with risky contraceptive use patterns can inform programs and policies designed to reduce levels of unintended pregnancy. METHODS A nationally representative sample of 1,978 adult women at risk for unintended pregnancy was surveyed over the telephone in 2004. Respondents provided information on contraceptive use over the past 12 months. Multiple logistic regressions were used to identify factors associated with different contraceptive use patterns. RESULTS Ambivalence about avoiding pregnancy was strongly associated with both contraceptive nonuse and having a gap in use while remaining at risk of unintended pregnancy (odds ratios, 2.4 and 2.0, respectively). Other significant predictors of either of these risky contraceptive behaviors were having less than a college education, being black, being 35-44 years old, having infrequent sexual intercourse, not being in a current relationship, being dissatisfied with one's method and believing that contraceptive service providers were not available to answer method-related questions (1.7-3.8). CONCLUSIONS Providers could better help women avoid unintended pregnancy by initiating regular assessments of method use difficulties, improving counseling on method choice and pregnancy risk, and identifying and assisting women at higher risk for inconsistent method use because of disadvantage, relationship characteristics or ambivalence about pregnancy prevention. In addition to providers' efforts, broader societal commitment is critical for increasing contraceptive knowledge and expanding access to contraceptive care for all women who are at risk of having an unintended pregnancy.