Although studies observe heterogeneity in the effects of adolescent childbearing on schooling, little is currently known about when this pattern emerged or how it changed across cohorts of women who lived in distinct periods of US history. This article identifies the potential origins of effect heterogeneity in the educational costs of adolescent childbearing and extends recent advances in causal inference to detect group differences in heterogeneity. The analysis applies this approach to four cohorts of women from the National Longitudinal Surveys who entered adolescence before, during, and after expansive economic, demographic, and cultural change in the 20th century. Results suggest that the educational costs of adolescent childbearing, as well as heterogeneity in those costs, increased for women in the latter half of the 20th century, especially for millennial women born 1980 84. The authors conclude that midcentury social changes fundamentally altered the educational costs of adolescent childbearing for women.