This paper documents an externality from the agricultural use of the most widely applied herbicide in the world—glyphosate—on birth outcomes of surrounding populations. We focus on the subclinical effects of water contamination in areas distant from the original locations of application. Our identification relies on: (i) the regulation allowing the introduction of genetically modified seeds in Brazil; (ii) the potential gain in municipality-level productivity from adoption of genetically modified soybean seeds and the strong complementary between these seeds and glyphosate; and (iii) the direction of water flow within water basins. We document a significant deterioration in birth outcomes for populations downstream from locations that are likely to have increased relatively more the use of glyphosate. According to our preferred specification, the average increase in glyphosate use in the sample during 2000–10 period led to an increase of 5% of the average in infant mortality rate.