Objective: To examine associations between mode of delivery and maternal mental health. Design: Prospective cohort study. Setting: Data from the Danish National Birth Cohort. Population: A total of 54,474 women were followed from diagnosis of pregnancy to 6 months postpartum. Methods: From interviews in pregnancy and 6 months postpartum, the women reported symptoms of anxiety, depression and stress. Mode of delivery was categorised as spontaneous vaginal delivery, instrumental vaginal delivery, planned caesarean section (CS) and emergency CS. Multiple linear and logistic regression models were used to compute absolute differences and odds ratios (ORs) with 95% confidence intervals (CIs) for associations between delivery mode and mental health indicators. Main outcome measures: Changes in combined score of emotional distress from pregnancy to 6 months postpartum. Presence of emotional distress and specific symptoms of anxiety, depression and stress 6 months postpartum. Results: Mental health indicators improved from pregnancy to 6 months postpartum for all modes of delivery. Improvement was strongest in mothers with planned CS and weakest in mothers with emergency CS. Six months postpartum, symptoms of overall emotional distress were more frequent in women with emergency CS (OR 1.21;1.06-1.37), as were specific symptoms of anxiety (OR 1.11;0.98-1.24), depression (OR 1.25;1.09-1.43) and stress (OR 1.14;1.01-1.29). Symptoms of anxiety were also more frequent in mothers with planned CS (OR 1.15;1.01-1.29). Conclusions: Mental health indicators improved from pregnancy to 6 months postpartum regardless of delivery mode. Mothers with emergency CS experienced more symptoms of emotional distress 6 months postpartum than mothers with spontaneous vaginal birth.