In developing countries, rainfall shocks around the time of birth have been shown decrease later health. The mechanism is unknown, but could run through income shocks, disease exposure, or increasing opportunity cost of parental time which influences parenting behavior. We use the Vietnam Demographic Health Surveys to study how rainfall shocks around the birth year influence a key dimension of parental behavior, breastfeeding. Consistent with the opportunity cost of time theory, rainfall reduces breastfeeding: 25% excess rainfall in the birth year decreases the proportion that is breastfed more than a year by 11 percentage points. The effect is particularly strong among farming families, for whom rainfall increases the opportunity cost of time through labor demand. These results are the first to shed light on the mechanism linking rainfall shocks and child health. Policy aiming to improve child health through breastfeeding should focus on the impact of labor demand on breastfeeding