Objective: We aimed to report ultrasound findings and the clinical course of transient abnormal myelopoiesis (TAM) in fetuses with Down syndrome.

Materials and Methods: Medical records of two cases of confirmed TAM in Down syndrome were retrospectively reviewed. We reviewed prenatal ultrasonographic findings, fetal blood analysis, flow cytometry, and the postnatal clinical course.

Results: From May 2010 to September 2013, two cases of TAM associated with Down syndrome were confirmed. Sonographic presentations of non-immune hydrops fetalis initially manifested late in the second trimester to the early third trimester, including fetal ascites, hepatomegaly, and cardiomegaly. Congenital infection was precluded. A complete blood count from cord blood in both cases showed abnormal leukocytosis with blast cells and fetal anemia. The platelet count showed thrombocytosis in one patient and thrombocytopenia in the other patient. Preterm birth was the only adverse obstetric outcome found in both cases. TAM spontaneously resolved after birth.

Conclusion: Fetal TAM is a hematological condition causing non-immune hydrops fetalis in fetuses with Down syndrome. Liver failure and anemia-induced high-output heart failure could be the pathogenesis of hydrops.