An increasing proportion of preterm births result from medical interventions, and the practice of aggregating all preterm births vs. splitting into spontaneous and medically indicated preterm births is inconsistent. While mechanistic and clinical arguments can be offered for either approach, we empirically evaluated the predictiveness of a range of risk factors for preterm birth in the Pregnancy, Infection, and Nutrition Study. Most influences were shared across the two subsets - African-American ethnicity, advancing age, delivery at a university medical centre, prior preterm birth and smoking. Medically indicated preterm births appeared to be associated with intensity of medical care, higher in the university medical centre and lower for the poorest women. Body mass index was positively associated with medically indicated preterm birth and inversely with spontaneous preterm birth. Given the complexity of the aetiological pathways, both aggregation and disaggregation are well justified and should be included in studies of the causes of preterm birth.