OBJECTIVES: We determined whether and to what extent a woman's exposure to stressful life events prior to conception (PSLEs) was associated with preterm birth and whether maternal age modified this relationship. METHODS: We examined 9350 mothers and infants participating in the first wave of the Early Childhood Longitudinal Study, Birth Cohort, a nationally representative sample of US women and children born in 2001, to investigate the impact of PSLEs on preterm birth in the United States. We estimated the effect of exposure on preterm birth with weighted logistic regression, adjusting for maternal sociodemographic and health factors and stress during pregnancy. RESULTS: Of the women examined, 10.9% had a preterm birth. In adjusted analyses, women aged 15 to 19 years who experienced any PSLE had over a 4-fold increased risk for having a preterm birth. This association differed on the basis of the timing of the PSLE. CONCLUSIONS: Findings suggest that adolescence may be a sensitive period for the risk of preterm birth among adolescents exposed to PSLEs. Clinical, programmatic, and policy interventions should address upstream PSLEs, especially for adolescents, to reduce the prevalence of preterm birth and improve maternal and child health.