["The Japanese Economic Review, EarlyView. ", "\nWe estimate the effect of prenatal exposure to radiation on infant health. By exploiting the 1983 Taiwanese radiation‐contaminated buildings accident as a natural experiment, we compare birth outcomes between siblings and cousins exposed to different radiation levels. Given the 1983 accident was unanticipated and exposed cohorts were unaware of the risk until 1992, our design isolates the effect of radiation exposure during pregnancy from other effects. We provide the first evidence that prenatal exposure to a continuous low‐level dose of radiation significantly reduces gestational length and increases the probabilities of prematurity and low birth weight.\n"]