OBJECTIVE: To study in a large-scale cohort with prospective data the associations of psychosocial adversities during pregnancy with length of gestation and offspring size at birth. METHODS: We defined a priori two types of psychosocial adversity during pregnancy: life stress (perceived burdens in major areas of life) and emotional symptoms (e.g. anxiety). Measures of offspring size at birth, including body weight, body length, abdominal and head circumference, were obtained from a national medical birth registry. We included in the analyses gestational age and offspring size at birth controlled for length of gestation; the latter was calculated by gestational-age-specific z scores (ZS) reported in 10(-3). We conducted multiple regression analyses adjusted for potential confounders to estimate the association between exposures and birth outcomes (n = 78017 pregnancies). RESULTS: Life stress (per score increase by 1; range, 0-18) was associated with shorter length of gestation (days; B, -0.14; 95% confidence interval (CI), -0.19, -0.10), increased offspring body weight (ZS; B, 9.14; 95% CI, 4.99, 13.28), body length (ZS; B, 6.58; 95% CI, 2.39, 10.77), abdominal circumference (ZS; B, 9.96; 95% CI, 5.77, 14.16), and head circumference (ZS; B, 6.13; 95% CI, 1.95, 10.30). Emotional symptoms were associated with shorter length of gestation (days; B, -0.04; 95% CI, -0.07, -0.004) and decreased body length (ZS; B, -4.44; 95% CI, -7.57, -1.32) only. CONCLUSIONS: Life stress and emotional symptoms both predicted a shorter length of gestation, while only life stress predicted an increased offspring size at birth controlled for length of gestation; yet, the associations were rather small. The fetoplacental-maternal unit may regulate fetal growth according to the type of psychosocial adversity and even increase fetal growth in response to maternal stress in major areas of life. This potentially reflects a basic principle of intrauterine human development in response to stress.