Transient abnormal myelopoiesis in neonates with Down syndrome is an unusual leukemia that spontaneously regresses within several months of life and is thought to arise in the fetal liver. It is largely unknown how the leukemic blasts proliferate and differentiate in fetal tissues. We report the histopathological findings of an autopsy case of a stillbirth with transient abnormal myelopoiesis. Blood vessels in almost all organs were filled with immature leukemic cells, most of which expressed megakaryocyte antigen CD42b. In contrast, leukemic cells infiltrating the tissues, including the pericardium, expressed myeloperoxidase. These findings indicate that leukemic progenitors in transient abnormal myelopoiesis can differentiate along both megakaryocytic and myeloid lineages, which may be influenced by microenvironmental factors. Numerous dysplastic mature/immature megakaryocytes and blasts were present in the liver, whereas the bone marrow contained predominantly myeloid cells at various stages of differentiation, suggesting that the fetal liver is the major organ for proliferation of blasts in transient abnormal myelopoiesis.

We present a case of transient abnormal myelopoiesis (TAM) presenting as non-immune fetal hydrops (NIHF). Hydrops fetalis (HF) is a condition associated with very high perinatal mortality, especially when no treatable cause, such as fetal anaemia, exists. In fetuses prior to 24 weeks with NIHF, a chromosomal anomaly is a common association. TAM is a leukaemic condition, almost entirely limited to children with Down syndrome. The presentation of TAM prenatally is unusual but cases may present ultrasonographically with NIHF and associated fetal hepatosplenomegaly. We report a case presenting in this manner with NIHF detected at 29 weeks\' gestation and discuss the subsequent diagnosis and management of in utero TAM.

The authors recently encountered a lethal case of Down syndrome with transient abnormal myelopoiesis (TAM). Although the peripheral white blood cell count and blast cells had improved without specific treatment, the patient died of severe coagulopathy due to liver fibrosis when he was 5 years old. The prognosis of TAM with liver fibrosis was poor. The patient had high levels of N-terminal peptide of III procollagen, type IV collagen, and hyaluronic acid. These serum makers are noninvasive indicators of liver fibrosis and may be useful as prognostic indicators of TAM in Down syndrome.