Objective: To search for transmission of hepatitis C virus (HCV) from infected mothers to their infants. Design: Prospective clinical, serologic, and molecular biologic follow-up (at least 3 months) of the infants of mothers with anti-HCV antibody. Setting: A county hospital providing primary and referral care in high-risk obstetrics (perinatology). Patients: Twenty-three mothers with anti-HCV antibody and their 24 infants. Methods: An enzyme-linked immunosorbent assay (EIA) and a four-antigen recombinant immunoblot assay (RIBA) were used to test for anti-HCV antibody; serum HCV RNA was measured in two independent laboratories by reverse transcription and polymerase chain reaction (PCR) using nested primers in the 5'-noncoding region. Infant samples were tested for HCV RNA by PCR at delivery and after 3 to 6 months of follow-up. Each sample was tested at least four times in two independent laboratories. Results: Twenty-nine of 648 mothers (4.5%; 95% CI, 3.0% to 6.4%) had anti-HCV antibody; these women had 30 babies. Twenty-three mothers and their 24 babies were followed at least 3 months (mean follow-up, 52 weeks). Of the 23 mothers, 21 (91%; CI, 72% to 99%) had a reactive RIBA; one woman had an indeterminate RIBA and was positive for HCV RNA by PCR. In 16 of 23 mothers (70%; CI, 47% to 87%), PCR yielded a positive result in both laboratories. The mean maternal alanine aminotransferase (ALT) level was 1.6 times the normal value. All the babies had anti-HCV antibody in cord-blood samples, but antibody disappeared or diminished in strength in interval samples, and no infant had evidence of active production of anti-HCV antibody. Only 1 of 24 (4%; CI, 0.1% to 21%) cord-blood samples was HCV RNA positive, and none of 24 (0%; CI, 0% to 14%) follow-up samples was positive for HCV RNA by PCR in either laboratory. Four mothers and one baby had antibody to HIV. Conclusions: Infant anti-HCV antibody is most likely acquired passively in utero, and vertical transmission of HCV is uncommon.
