Our findings suggest that the increased risk of preterm delivery among low income women is primarily due to the etiology of preterm contractions, rather than their access to or quality of care after the onset of contractions. Income supplementation or changes to public health insurance programs are unlikely to reduce preterm delivery among women who have preterm contractions. These factors associated with preterm contractions and birth appear to carry across births; women who delivered preterm or low birth weight before are much more likely to experience preterm contractions and to deliver preterm whether or not they experience contractions in the current pregnancy. Understanding and preventing spontaneous preterm contractions and delivery will require additional research into the factors associated with preterm contractions.