This paper uses birth records from California and mothers who move to quantify the absolute and relative importance of birth location in early-life health. Using a model that includes mother and location fixed effects, we find that moving from a below- to an above-median birth weight location leads to a 19-gram increase in average birth weight. These causal place effects explain 16 percent of geographic variation in birth weight, with family-specific factors accounting for the remaining 84 percent. Place effects are more influential for children of non-college-educated mothers, and are most strongly correlated with local levels of pollution. The improvement in birth weight from moving to a higher-quality area compares favorably to policies that target maternal health, and could have a small, lasting effect on long-run outcomes.