To assess the prevalence of hepatitis C virus (HCV) infection in pregnant women and the rate of vertical transmission in infected mothers belonging to a low-risk group, 1,388 women were tested for HCV antibody at delivery. Twenty-five anti-HCV-positive women with no apparent source of HCV exposure were recruited. A reverse transcriptase-polymerase chain reaction (RT-PCR) and a new quantitative branched DNA-based signal amplification assay (bDNA) were used to detect HCV RNA. The rate of anti-HCV positivity in pregnant women was 2.5% (36 of 1,388). Of the 25 cohort mothers, 18 (72%) were positive for HCV RNA by RT-PCR, 13 of whom were also positive by the bDNA assay (sensitivity 72.2%). Of the 25 infants of low-risk mothers tested at birth, 22 were anti-HCV positive, two were weakly reactive, one was negative, and none was viremic. Neither active humoral immunoresponse nor HCV RNA was detected in any of the infants over a period of 12 months. These data suggest a relatively high prevalence of anti-HCV in unselected pregnant women and a poor efficiency of vertical transmission of HCV in a low-risk population, irrespective of the viral burden of the mother-to-be.