Serial ultrasound scans were done in 150 fetuses between 14th to 22nd week of gestation to establish the nomograms of anterior ventricular hemisphere ratio (AVHR) and posterior ventricular hemisphere ratio (PVHR). Of 150 fetuses, 100 were in the high risk group for neural tube defect and 50 were in the control group. The study indicates that the value of AVHR decreases from 0.62 to 0.50 and PVHR from 0.60 to 0.50 between 14th to 22nd week of gestation. No statistical difference was observed in the values of AVHR and PVHR in high risk and low risk (control) cases (p > .001). The value of AVHR or PVHR greater than 0.5 after 18 weeks of gestation or more was considered pathological for hydrocephalus. In 2, out of 3 cases of hydrocephalus detected in our series, the value of AVHR and PVHR was 0.7 at 20 weeks and in the third case it was 0.6 at 18 weeks. All of these values were 3 SD above the normal for the period of gestation.