Objective: To investigate the association between maternal exposure to severe life events and fetal growth (birthweight and small for gestational age). Stress has been associated with adverse pregnancy outcome. Methods: Mothers of 1.38 million singleton live births in Denmark between January 1, 1979 and December 31, 2002 were linked to information on their spouses, parents, siblings, and older children. Exposure was defined as death or serious illness in a relative during pregnancy or in the 6 months before conception. Linear regression was used to examine the effect of exposure on birthweight. Log-linear binomial regression was used to assess the effect of exposure on small for gestational age. Results: Death of a relative during pregnancy or in the 6 months before conception reduced birthweight by 27 g (adjusted estimate -27 g, 95% Confidence Interval (CI = -33, -22). There was a significant association between maternal exposure to, death of a relative and risk of a baby weighing below the 10th percentile (adjusted relative risk (RR) = 1. 17, 95% CI = 1.13, 1.22) and 5th percentile (adjusted RR = 1.22, 95% Cl = 1. 15, 1.29). Conclusions: Mothers exposed to severe life events before conception or during pregnancy have babies with significantly lower birthweight. If this association is causal, the potential mechanisms of stress-related effects on birthweight include changes in lifestyle due to the exposure and stress-related dysregulation of the hypothalamic-pituitary-adrenal axis during pregnancy.