We aimed to clarify how maternal physical characteristics explains the association between adolescent pregnancy and adverse birth outcomes, focusing on their height. We used a national multicenter-based delivery registry among 30,831 women under age 25 years with a singleton pregnancy between 2005 and 2011. Adolescent pregnancy was defined as younger than 20 years of age, and categorized into "junior adolescent" (aged ≤15 years) and "senior adolescent" (aged 16-19 years). We used multivariate Poisson regression and mediation analysis to assess the extent to which maternal height explained the association between adolescent pregnancy and risk of adverse birth outcomes. Risks for preterm birth [(adjusted risk ratio (aRR) 1.17, 95% confidence interval (95% CI), 1.08-1.27], low birthweight (aRR 1.08, 95% CI, 1.01-1.15), and low Apgar score (aRR 1.41 95%CI, 1.15-1.73) were significantly higher among adolescent women compared to women of 20-24 years of age. The mediation effect of maternal height on these outcomes were moderate for low birthweight (45.5%) and preterm birth (10.5%), and smaller for low Apgar score (6.6%). In all analyses, we did not detect significant differences between junior adolescent and senior adolescent. Adolescent women have higher risk of adverse birth outcomes. This association is partially mediated by shorter maternal height.