AbstractBackgroundStudies assessing the association between short birth interval, a birth-to-birth interval of less than 33 months, and under-five undernutrition have produced inconclusive results. This study aimed to assess the relationship between short birth interval and outcomes of stunting, underweight, and wasting among children aged under-five in Ethiopia, and potential mediation of any associations by maternal anemia and baby birth size.MethodData from the 2016 Ethiopia Demographic and Health Survey (EDHS) was used. Stunting, wasting, and underweight among children aged under-five were outcome variables. Generalized Structural Equation Modeling (GSEM) was used to examine associations between short birth interval and outcomes, and to assess hypothesized mediation by maternal anemia and baby birth size.ResultsSignificant associations between short birth interval and stunting (AOR = 1.49; 95% CI = 1.35, 1.66) and underweight (AOR = 1.43; 95% CI = 1.28, 1.61) were found. There was no observed association between short birth interval and wasting (AOR = 1.05; 95% CI = 0.90, 1.23). Maternal anemia and baby birth size had a significant partial mediation effect on the association between short birth interval and stunting (the coefficient reduced fromβ = 0.337,p < 0.001 toβ = 0.286,p < 0.001) and underweight (the coefficient reduced fromβ = 0.449,p < 0.001 toβ = 0.338,p < 0.001). Maternal anemia and baby birth size mediated 4.2% and 4.6% of the total effect of short birth interval on stunting and underweight, respectively.ConclusionMaternal anemia and baby birth size were identified as mediators of the association between short birth interval and under-five undernutrition status. Policies and programs targeting the reduction of under-five undernutrition should integrate strategies to reduce maternal anemia and small baby birth size in addition to short birth interval.