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• Risk-based hepatitis C virus (HCV) screening in pregnant women contributes to missed diagnoses of HCV. • Inhaled drug use, HCV-positive partnerships, and having three or more lifetime partners were associated with HCV viremia. • Infant clearance of HCV is associated with maternal genetic polymorphisms. • Close follow-up of HCV pregnant women is imperative in Latin America. Porto Alegre, in south Brazil, has one of the highest hepatitis C virus (HCV) infection rates in the country (84.4 cases/100 000 in 2018). Prenatal screening of HCV, however, has not been routinely offered. A longitudinal study of pregnant women with HCV and their infants was conducted between January 2014 and December 2018. Screening for HCV antibodies was offered to all women delivering at the study tertiary institution. HCV RT-PCR was performed if the woman was seropositive. Infants were followed prospectively. Among 18 953 pregnant women delivering infants during the study period, 17 810 were screened for HCV antibodies (93.9%) with 130 positive results (HCV seroprevalence 0.7%). HCV-RNA was detectable in 57/117 cases (48.7%). HCV viremia was associated with the use of injectable drugs (P = 0.03), inhaled/crack drug use (P = 0.02), having an HCV-seropositive partner, and ≥3 lifetime sexual partners (P < 0.01). Genotype 1 was most prevalent (68%) during pregnancy. Among 43 children with follow-up, six (13%) were HCV-infected (transmission rate 13.9%); 50% were infected with genotype 3. Two infants (33%) cleared their infection; the mothers had genetic polymorphisms associated with clearance. HCV vertical transmission was high in the study population, with HCV infection during pregnancy being vastly underdiagnosed. Public health efforts must focus on this vulnerable population for disease prevention and early treatment. [ABSTRACT FROM AUTHOR]