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IntroductionSexually transmitted infection (STI) prevalence and incidence estimates for pregnant adolescents are under-reported. We estimated prevalence and incidence of STIs in pregnant adolescents (15–19 years) in comparison with pregnant women 20–24 and >25 years.MethodsPregnant women registering at primary care clinics in Umlazi, a periurban subdistrict in KwaZulu-Natal, South Africa, were enrolled in an HIV incidence cohort study during February 2017–March 2018. Women were examined for abnormal vaginal discharge, received empirical treatment, tested for HIV-1 and had vaginal swabs taken at their first and a subsequent visit in the third trimester. Vaginal swabs were stored for STI testing at completion of study and tested forTrichomonas vaginalis, Chlamydia trachomatis, Neisseria gonorrhoeaeandMycoplasma genitaliumusing PCR.ResultsA total of 752 HIV-negative pregnant women were enrolled at a median gestational age of 17 weeks: 180 (23.9%), 291 (38.7%) and 281 (37.4%) in the 15–19, 20–24 and >25 years age groups. Pregnant adolescents had an STI prevalence of 26.7% at baseline, not significantly lower than the 20–24 (34.7%, OR 1.4; 95% CI 1.0 to 2.1, p=0.09) and >25 years (33.8%, OR 1.4; 95% CI 0.9 to 2.1, p=0.12) age groups.T. vaginalis(11.1%),C. trachomatis(7.8%) andN. gonorrhoeae(4.4%) were most prevalent in adolescents, a trend similar to the other age groups. Overall, 43.4% were symptomatic and treated at baseline. Overall, 40.7% (118 of 290) of women who tested negative for an STI at baseline tested positive at the repeat visit (incidence 19.5/100 person years). STI incidence in pregnant adolescents was 23.9/100 person years and comparable with older age groups (20.5/100 person years and 16.2/100 person years). At the repeat visit, 19.0% of all women with an STI were symptomatic and treated. Performance of syndromic management was poor at baseline (negative predictive value (NPV) 68.6%, positive predictive value (PPV) 34.0%) and at repeat visit (NPV 58.4%, PPV 34.3%).ConclusionsPrevalence of asymptomatic curable STIs in pregnant adolescents is high and comparable with women >20 years old. Adolescents remain at substantial risk of asymptomatic incident STIs during pregnancy.