- Idiopathic hydramnios is an independent risk factor for perinatal morbidity and mortality, although the precise mechanism is unknown. Hydramnios may alter oxygen delivery to the human fetus. The middle cerebral artery (MCA) pulsatility index (PI) is a sensitive parameter for detection of blood flow redistribution or centralization. The aim of this study was to determine whether idiopathic hydramnios is associated with changes in the PI of the MCA and the umbilical artery (UA). The PI of the MCA and UA were determined prospectively in 113 consecutive pregnant women with singleton pregnancies and idiopathic hydramnios (study group) and in 113 matched control patients with normal amniotic fluid (AF) volume (control group). The control group was matched for maternal age, gravidity, parity and gestational age at examination. Hydramnios was defined as an AF index above 24 cm. Patients with fetal structural or chromosomal anomalies and with diabetes mellitus were excluded. A PI below the 5th centile for the MCA was considered abnormal. The rate of abnormal MCA PI was significantly higher in the study group than in the control group (30.9% (35/113) vs. 5.3% (6/113), respectively, OR 8.0; p < 0.0001; 95% CI 3.3-22.1). A significant inverse correlation was found between MCA PI and increasing AF index among patients with hydramnios (p = 0.008, r = 0.46). In contrast, there were no significant differences between the UA PI (0.93 (0.57-2.06) vs. 1.0 (0.49-1.48), (p = 0.20)) and MCA/UA (1.86 (0.77-2.01) vs. 2.15 (0.86-4.11), (p = 0.14)) in both groups. Our observations provide evidence of cerebral blood flow redistribution in fetuses with idiopathic hydramnios.