While the effects of low birth weight have long been explored, the literature on the effects of high birth weight is sparse. However, with increasing obesity rates in the United States, high birth weight has become a potential concern, and has been associated in the medical literature with an increased likelihood of becoming an overweight child, adolescent, and subsequently an obese adult. Overweight and obesity, in turn, are associated with a host of negative effects, including lower test scores in school and lower labor market prospects when adults. If studies only focus on low birth weight, they may underestimate the effects of ensuring that mothers receive adequate support during pregnancy. This study finds that cognitive outcomes are adversely affected not only by low birth... weight (<2500 grams) but also by high birth weight (>4500 grams). Our results have policy implications in terms of provision of support for pregnant women.