This paper examines the causal effects of family size and demographic structure on offspring's international migration. We use rich survey data from Mexico to estimate the impact of sibship size, birth order and sibling composition on teenagers' and young adults' migration outcomes. We find no empirical support for the hypothesis that high fertility drives migration. The positive correlation between sibship size and migration disappears when endogeneity of family size is addressed using biological fertility (miscarriages) and infertility shocks. Yet, the chances to migrate are not equally distributed across children within the family. Older siblings, especially firstborn males, are more likely to migrate, while having more sisters than brothers may increase the chances of migration, particularly among girls.