Pregnancy outcomes in the United States are generally worse than those in most developed countries. Contributing to these adverse outcomes are the relatively high levels of poverty in the United States, a characteristic that is associated with decreased utilization of appropriate prenatal care and delivery services as well as having an increased number of other risk factors. Poor women tend to be more obese, to have more medical conditions, such as hypertension and diabetes, to be more likely to be stressed or depressed, and to smoke cigarettes and use illicit drugs. We present some of the potential mechanisms that explain the association between these characteristics and adverse pregnancy outcomes--focusing on preterm birth.