We show that children who are born at or just before the weekend are less likely to be breastfed, owing to poorer breastfeeding support services in hospitals at weekends. We use this variation to estimate the effect of breastfeeding on children's development in the first seven years of life, for a sample of births of low-educated mothers. We find large effects of breastfeeding on children's cognitive development but no effects on health or noncognitive development during the period of childhood we consider. Regarding mechanisms, we study how breastfeeding affects parental investments and the quality of the mother-child relationship. (JEL I12, I14, I18, J13, J16, J24)