Placental hydropic change seen here in the chorionic villi is accompanied by increased fetal erythroblasts and nucleated RBCs in fetal vessels in villi at the upper left. These changes can accompany any cause for fetal anemia, whether immune (erythroblastosis fetalis from Rh incompatibility, or other maternal antibody directed at fetal RBCs) or non-immune. Non-immune causes include infections, genetic disorders, and many others. Perhaps a fourth to a third of the time, no cause for hydrops is readily apparent. In this case, the fetus has a hemoglobinopathy: alpha thalassemia major, leading to hydrops (the most common cause for hydrops in persons of Southeast Asian ancestry).

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