This chapter focuses on pathogenic organisms that may be responsible for fetal infection during pregnancy and may have significant effects on outcome. Many infections have associated serious consequences including fetal/perinatal mortality and significant morbidity. Infection with parvovirus can cause fetal hydrops and/or cardiac involvement. Prenatal diagnosis of fetal parvovirus infection requires samples of fetal blood, serous body effusions, and amniotic fluid to be sent to recognized reference laboratories. Chickenpox in adults is often more symptomatic and leads to the risk of varicella pneumonia, which carries a significant mortality. Diagnosis of primary cytomegalovirus (CMV) infection in pregnant women is based on serological testing with the appearance of specific IgG. The main host cells infected by CMV are the endothelial cells and the polymorphic nuclear leukocytes. CMV can be detected in the amniotic fluid by conventional viral isolation, rapid culture, or molecular assays.