We report a case of a male newborn with trisomy 21 and transient abnormal myelopoiesis at birth whose placenta showed extravasated fetal blasts in the perivillous (maternal) space. Concern for possible maternal spread of fetal malignancy prompted a Kleihauer-Betke test and flow cytometric analysis of the maternal peripheral blood on postpartum day 2. Notably, no evidence of the persistence of fetal cells in the maternal blood was identified, a finding that likely reflected successful maternal immunologic clearance of the fetal blasts and erythrocytes, and/or blast cellular fragility and limited viability. Ours is the first report, to our knowledge, documenting maternal peripheral-blood follow-up evaluation of this disorder in the English literature. We discuss our case in the context of a comprehensive review of fetoneonatal solid tumor and leukemic proliferative disorders with placental involvement and evidence of maternal metastasis.