Why is human parvovirus infection important to a pregnant woman?

Human parvovirus B19, the etiologic agent of erythema infectiosum, readily infects erythroblasts and may, therefore, result in hydrops fetalis and fetal death. This risk is small, with current studies showing a 2% to 9% risk after infection during the first 16 to 28 weeks of pregnancy. Fortunately, about 50% of pregnant women have serologic evidence of prior exposure to parvovirus B19.

Tolfvenstam T, Broliden K: Parvovirus B19 infection, Semin Fetal Neonatal Med 14:218–221, 2009.