Background:N-terminal pro-B-type natriuretic peptide (nt-proBNP) is an established marker of heart failure in adult cardiology. We analyzed nt-proBNP in the circulation of fetuses with increased volume load secondary to anemia and investigated the effect of treatment on nt-proBNP concentration.Methods:Fetuses undergoing intrauterine transfusion (IUT) were examined. Nt-proBNP was measured before IUT and correlated with hemoglobin concentrations, ultrasonographic findings and Doppler measurements of the peak systolic velocity of the middle cerebral artery (MCA-PSV).Results:27 patients (seven with hydrops) and 78 controls were examined. Nt-proBNP was markedly elevated in anemia (p < 0.001). Concentrations were highest in hydropic fetuses (p < 0.03); no difference was present in hemoglobin and MCA-PSV values between hydropic and non-hydropic cases. In fetuses undergoing multiple IUTs nt-proBNP normalized after the third IUT, whereas hemoglobin and MCA-PSV remained abnormal.Conclusions:Levels of circulating nt-proBNP correlate well with the degree of myocardial work load in the hyperdynamic state of fetal anaemia. We hypothesize that normalization of nt-proBNP after serial transfusions is an indicator of myocardial adjustment to chronic anemia. Nt-proBNP measurement may be useful in the management of fetal anaemia, particularly in cases at risk of hydrops and fetuses requiring multiple transfusions.