In order to translate research findings into effective prevention strategies, it is important to understand people's beliefs about the causes of poor health outcomes. However, with the exception of knowledge and beliefs about folic acid supplementation, little is known regarding women's causal attributions women regarding birth defects. We employed Attribution Theory constructs to analyze open-text interview responses from 2,672 control mothers in the National Birth Defects Prevention Study who gave birth in 1997-2005. Common themes included use of alcohol, tobacco, illicit drugs, and medications during pregnancy. Stress and emotional upset were also suggested as possible causes of birth defects. Genetic- and heredity-related responses were more likely to be mentioned by Asian/Pacific Islander women compared to non-Hispanic Whites. Hispanic women were less likely to suggest several specific possible teratogens, such as paint, pesticides, or other chemicals, but were more likely to suggest events occurring during childbirth. Differences also emerged among ethnic groups for theoretical constructs, although most responses were categorized as controllable, changeable over time, and with an internal locus of causality.