There is a growing consensus that emissions of greenhouse gases due to human activity will alter the earth’s climate, most notably by causing temperatures, precipitation levels, and weather variability to increase. The design of optimal climate change mitigation policies requires estimates of the health and other benefits of reductions in greenhouse gases; current evidence on the magnitude of the direct and indirect impacts, however, is considered insufficient for reliable conclusions (A. J. McMichael et al. 2003).1 In addition to the overall predicted warming trend, one important feature of many global climate change forecasts is an increased incidence of very high and low temperatures. To provide evidence of the potential benefits of greenhouse gas reductions, this paper documents whether the temperature variation predicted to be part of climate change, including extreme high and low temperatures, historically has had negative health consequences through its effect on babies while in utero. Using individual-level data on more than 37.1 million births, we find that exposure to extreme hot temperatures during pregnancy leads to lower birth weight. We combine this