OBJECTIVE: To study the association between fetal middle cerebral artery flow and the lateral cerebral ventricular width throughout gestation. METHODS: The study is a prospective cohort evaluation of 430 singleton male and female fetuses between 20 and 40 weeks' normal gestation. Abdominal ultrasonography and Doppler measurements were performed to measure the fetal atrial ventricular size and resistance index (RI) of the middle cerebral artery. RESULTS: The mean (± standard deviation) ventricular width was 6.2 ± 1.2 mm. The modification in the RI of the middle cerebral artery throughout gestation showed a biphasic mode, increasing gradually to a peak at 30 weeks' gestation and decreasing progressively thereafter. No significant correlation was found between the middle cerebral artery flow and the lateral cerebral ventricular width (r = .11). In addition to the 430 cases studied, three cases of mild ventriculomegaly and three cases of hydrocephalus were evaluated. The RI of the middle cerebral artery was within the normal range in all six of these cases. CONCLUSION: Fetal middle cerebral artery blood flow is not affected by the width of the lateral ventricles, even in enlarged ventricles.