The AoI is complex and its variations are difficult to understand. Anatomically is important as it is located between the cerebral and the placental. Retrospective analysis from US databases. Images with AoI, umbilical artery PI, MCA PI were retrieved. AoI velocities were measured as follows: 1: peak systolic; 2: systolic notch, either positive or negative if inverted; 3: peak diastolic, as the highest velocity during diastole; 4: end diastolic, the lowest velocity during diastole. All these were studied and correlated to UA, MCA, CPR, fetal weight z-score and GA. There were 197 scans, from 22 to 42 weeks, @mean 32 weeks. AoV1 (systolic peak) was correlated with greater GA (R = 0.25, p < 0.001). Aov2 (systolic notch) lowered progressively during gestation (R = -0.44 p < 0.001). Diastolic velocities (v3 & v4) did not correlate to GA. AoV4 were not correlated to UA PI, MCA PI nor CPR. In fetuses ≥ 30 weeks (n = 124), Aov3 was positively correlated to CPR (Spearman R = 0.18 p < 0.05), and MCA PI (R = 0.18 p < 0.05). This finding supports that whenever the MCA diastole is normal, (elevated PI), the diastolic flow in the AoI is higher, toward the placenta, in fetuses @30w and later. There is a regulation of cerebral resistance and aortic flow that could be detected in normal adequate growing fetuses. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.