Fire has long served as a tool in agriculture, but the practice's link with economic activity has made its health consequences difficult to study. Drawing on data from satellite-based fire detection systems, air monitors, and vital records in Brazil, we study how in utero exposure to smoke from sugarcane harvest fires affects health at birth. Exploiting daily changes in fire location and wind direction for identification, we find that late-pregnancy smoke exposure decreases birthweight, gestational length, and in utero survival. Fires less associated with smoke exposure predict improved health, highlighting the importance of disentangling pollution from its economic correlates.