prematurity is a leading cause of neonatal mortality and a global health problem that affects high, middle and low-income countries. several factors may increase the risk of preterm birth. in this article, we test the hypothesis that different risk factors determine preterm birth in different income groups by investigating whether risk factors for preterm deliveries in the 2004 pelotas (rio grande do sul state, brazil) birth cohort vary among those groups. a total of 4,142 women were included in the analysis. preterm births were equally common among women who had spontaneous vaginal deliveries as for those with induced or operative births. in the multivariate analysis the factors that remained significantly associated with preterm birth were black skin color, low education, poverty, young maternal age, primiparity, previous preterm birth, inadequacy of prenatal care and reported hypertension. in the analyses repeated after stratification by family income terciles, there was no evidence of effect modification by income and no clear difference between the socioeconomic groups. no association between cesarean section and preterm delivery was found. further studies are required to understand the causes of the epidemic of preterm births in brazil.