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Objectives: Facilitate risk assessment of vital complications in fetuses of pregnancies affected by acute parvovirus B19 (B19V) infection. Design: Study of the natural course of maternal B19V infection in four cases, from early pregnancy on. Setting: University Medical Center in the Netherlands. Population: Pregnant mothers attending obstetric services. Methods: Serial measurements of the maternal and fetal or neonatal viral load and antibody responses. Main Outcome Measures: Maternal and fetal/neonatal serum B19V viral DNA load and specific IgM and IgG antibodies in maternal serum. Results: Peak viral load levels occurred within 1 week after maternal infection and peak IgM levels were observed 1 week after the peak viral load levels. Approximation of IgG and IgM ratios usually took place 4 weeks after infection. Vertical transmission occurred 1–3 weeks after maternal infection, suggesting that fetal infection occurs during the maternal peak viral load. Conclusions: Maternal B19V DNA load levels and IgM responses are useful to estimate the risk of parvovirus B19-associated fetal complications. The maternal peak viral load directly precedes the onset of fetal infection and may be used to indicate the stage of intrauterine B19V infection. Copyright © 2007 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]