IntroductionAdolescent pregnancy has received little attention, despite being it is the leading cause of maternal mortality and morbidity in low-income countries, including Ethiopia. Sidama regional state is one of the regions in Ethiopia that has high rates of adolescent pregnancy. This study aimed to identify factors related to adverse outcomes of adolescent pregnancy, in the Sidama region, Ethiopia. MethodsA hospital-based unmatched case-control study was conducted among 120 cases and 240 controls of pregnant adolescents. Structured interviews and patient record reviews were used to examine the potential correlates and adverse outcomes of adolescent pregnancy. Using binary logistic analysis, adjusted odds ratio (AOR) with 95% confidence intervals, were computed to identify factors related to adverse outcomes of adolescent pregnancies. ResultsA monthly income below the poverty level (AOR: 3.40; 95% CI, 1.21–9.58), lack of antenatal care follow-up (AOR: 4.22; 95% CI, 1.97–9.04), experiencing gender-based violence (AOR: 2.03; 95% CI, 1.16–3.57), and referral to a specialized health facility (AOR: 2.79; 95% CI,1.39, 5.62) were associated with higher odds of adverse pregnancy outcomes. ConclusionSeveral socio-economic and health care system-related determinants are associated with adverse outcomes of adolescent pregnancy. Therefore, it is crucial to improve free and accessible maternal health care services for adolescents, focusing on education, challenging social norms that condone gender-based violence as well as enhancing the referral system to lessen the burden of adverse outcomes of adolescent pregnancy.