Transient myeloproliferative disorders can be associated with hydrops in Down syndrome fetuses. No cases of prenatal management of such a condition have been reported in the literature. We report a case of myeloproliferative disorder diagnosed by cordocentesis at 31 weeks in a Down syndrome fetus with pericardial effusion. A pericardiocentesis was performed at the first signs of hydrops and successfully improved fetal cardiac function, allowing for continuation of pregnancy.