An isolated pericardial effusion was observed during a routine prenatal ultrasound in a fetus of 30 and 3/7 weeks gestation. Amniocentesis was performed and revealed a trisomy 21. After prenatal counseling, the parents opted for termination of the pregnancy at 32 weeks. Postmortem examination confirmed the presence of a pericardial effusion, without structural cardiac anomalies, and showed the development of ascites and subcutaneous edema. Histological examination showed an infiltrate of megakaryoblasts and irregular, dysplastic megakaryocytes confined to the epicardium, the pericardial lymph nodes, and the pancreas, consistent with a myeloid proliferation related to Down syndrome. Sequencing of exons 2 and 3 of the GATA1 gene from the umbilical cord blood and from megakaryoblast infiltrate showed no mutation. A high incidence of chromosomal abnormalities, in particular trisomy 21, has been described in fetuses with pericardial effusion. However, myeloid proliferation related to Down syndrome without GATA1 mutations is extremely rare. To our knowledge, only one such case has been reported to date. We present here a 2nd case, which further supports the hypothesis that hyperproliferation of megakaryocytes in a subset of Down syndrome patients may be initiated by events other than GATA1 mutations.