Objective. Preterm birth is an important indicator of neonatal wellbeing. Infants born preterm are at higher risk for severe morbidity and mortality. Apart from medical risk factors, social factors are also associated with preterm birth. This study aims to provide knowledge on factors which have a predictive role in relation to preterm birth. Design. Cross-sectional population study. Setting. The Brussels metropolitan region. Population. Women who gave birth in the Brussels metropolitan region in 2004. In total, 8,586 birth registration forms were reviewed for this study. Main outcome measures. Associations between educational level, occupation, marital status, age, and origin with preterm birth (<37 weeks). Methods. After bivariate analyses, significant correlations were explored in a stepwise logistic regression model. Odds ratios (OR) were calculated for each of the significant characteristics in the final model. Results. The risk of preterm birth was found to be associated with age, origin, and marital status. After controlling for confounding factors, the two most important social risk factors for preterm birth were being a teenage mother (OR 2.15, 95% CI: 1.31–3.53) or single mother (OR 1.49, 95% CI: 1.17–1.91). Conclusions. Being a teenage or single mother are important social risk factors for preterm birth. We hypothesize that the increased risk is related to prenatal care trajectories. To obtain a better understanding of preterm birth occurrences, further research should focus on mapping these trajectories and specifically targeting the most vulnerable groups.