Objectives. We sought to determine if and to what extent a woman's exposure to stressful life events prior to conception (PSLEs)were associated with subsequent infant birth weight by using a nationally representative sample of US women. Methods. We examined 9350 mothers and infants participating in the first wave of the Early Childhood Longitudinal Study, Birth Cohort in 2001. Weighted regressions estimated the effect of exposure on very low and low birth weight, adjusting for maternal sociodemographic and health factors and stress during pregnancy. Results. Twenty percent of women experienced any PSLE. In adjusted analyses, exposed women were 38% more likely to have a very low birth weight infant than nonexposed women. Furthermore, the accumulation of PSLEs was associated with reduced infant birth weight. Conclusions. This was the first nationally representative study to our knowledge to investigate the impact of PSLEs on very low and low birth weight in the United States. Interventions aimed to improve birth outcomes will need to shift the clinical practice paradigm upstream to the preconception period to reduce women's exposure to stress over the life course and improve the long-term health of children.