OBJECTIVES: To assess the prevalence of risk factors for adverse pregnancy outcome during the preconception stage and during pregnancy, and to assess differences between women in preconception and pregnancy. METHODS: Data from the 2002 and 2004 Behavioral Risk Factor Surveillance System, United States, were used to estimate the prevalence of selected risk factors among women 18-44 in the preconception period (women who wanted a baby in the next 12 months, and were not using contraception, not sterile and not already pregnant) with women who reported that they were pregnant at the time of interview. RESULTS: Major health risks were reported by substantial proportions of women in the preconceptional period and were also reported by many pregnant women, although pregnant women tended to report lower levels of risk than preconception women. For example, 54.5% of preconception women reported one or more of 3 risk factors (frequent drinking, current smoking, and absence of an HIV test), compared with 32.0% of pregnant women (p < .05). The difference in the prevalence of these three risk factors between preconception and pregnancy was significant for women with health insurance (52.5% in preconception vs. 29.4% in pregnancy, p < .05), but not for women without insurance (63.4% vs. 52.7%, p > .05). CONCLUSIONS: Women appear to be responding to messages regarding behaviors that directly affect pregnancy such as smoking, alcohol consumption and taking folic acid, but many remain unaware of the benefits of available interventions to prevent HIV transmission and birth defects. Although it appears that some women reduce their risk for adverse pregnancy outcomes after learning of their pregnancy, the data suggest that a substantial proportion of women do not. Furthermore, if such change occurs it is often too late to affect outcomes, such as birth defects resulting from alcohol consumption during the periconception period. Preconception interventions are recommended to achieve a more significant reduction in risk and further improvement in perinatal outcomes.