Parvovirus B19 is the viral agent that causes the childhood exanthum erythema infectiosum, or fifth disease. Approximately 50% of pregnant women are seropositive for this agent and thus immune to primary infection. However, acute infection may develop in seronegative pregnant women exposed to B19. Acute B19 infections during pregnancy have been associated with miscarriage and hydrops fetalis. This latter condition is amenable to fetal therapy via intrauterine transfusion.