Objective: The purpose of this study was to evaluate the fetal vascular adaptation to moderate and severe maternal anemia. Study Design: Biometry; amniotic fluid index; uterine, cerebral, and umbilical Doppler; and maternal hemoglobin level were measured at admission and 8 days after treatment. Results: Group 1 consisted of 16 pregnancies (maternal hemoglobin level, 6.9 ± 0.6 g/100 mL); group 2 consisted of 23 pregnancies (maternal hemoglobin level, 5 ± 0.6 g/100 mL). At admission the cerebral and cerebral/umbilical Doppler indexes, amniotic index, and biometry were lower in group 2. The uterine index was normal in both groups. An abnormal fetal heart rate was found in group 2 only (48%). At day 8, maternal hemoglobin level and amniotic index increased more in group 2 than in group 1. The cerebral index and the cerebral-to-umbilical resistance ratio increased only in group 2. The abnormal fetal heart rate disappeared in group 2. Conclusion: Only severe maternal anemia (maternal hemoglobin level, <6 mg/L) triggered fetal cerebral vasodilation and reduced amniotic volume. (Am J Obstet Gynecol 2003;188:794-9.)