Background: Adolescents are disproportionally affected by sexually transmitted infections (STI). Chlamydia trachomatis (CT) and Trichomonas vaginalis (TV) are the most frequent curable STI in adolescents, causing serious consequences for their reproductive health. Therefore, we aimed to determine the prevalence and incidence of CT and TV, as well as their risk factors in pregnant adolescents from Belém, northern Brazilian Amazon. Methods: This prospective study enrolled 199 adolescents up to 20 weeks of pregnancy. They were scheduled for follow-up visit between 28 and 29 weeks of pregnancy. Sociodemographic and behavioral data were obtained by interview. Cervicovaginal samples were taken to test for TV, CT, Neisseria gonorrhoeae and bacterial vaginosis. Univariate and multivariate analyses were performed to test the association of prevalent/incident CT and TV with the variables. Results: Prevalence of cervical CT infection was 33.7% (n = 67/ 199), and for trichomoniasis it was 4.0% (n = 8/199). Cervical ectopy increased the risk for prevalent CT (OR, 1.93; 95% CI, 1.01-3.70), while having treated vaginal discharge in the past (OR, 0.51; 95% CI, 0.26-0.98) and being married (OR, 0.10; 95% CI, 0.01-0.83) were protective against * Corresponding author. C. Marconi et al. 678 current CT and TV, respectively. Among the 95 (47.7%) adolescents who completed follow-up, 15 cases of incident CT were identified. Incident CT was associated with having a formal or informal job (OR, 28.4; 95% CI, 2.1-391.6) and bacterial vaginosis treatment at the baseline (OR, 0.08; 95% CI, 0.01-0.69). Conclusion: Prevalence and incidence rates of TV and CT are high in this population devoid of STI routine screening. Treatment of bacterial vaginosis may benefit this population by reducing risk for CT acquisition.