To assess the usefulness of transverse cerebellar diameter (TCD) as an independent parameter for gestational age assessment in third trimester of pregnancy.An observational study.Diagnostic Radiology Department, PNS Shifa, Karachi between April 2002 to July 2004.This study was performed on patients between 26-38 weeks of gestation with normal pregnancy. Grey scale antenatal ultrasonography was done with 3.5 MHz probe to measure fetal transverse cerebellar diameter and was correlated with gestational age in weeks. These cases were followed progressively at different periods of gestation throughout the pregnancy. Collected data was converted into variables which were analyzed by SPSS version 10, descriptive statistics included mean +/- standard deviation which were computed for TCD and other fetal biometry parameters.Transverse cerebellar diameter varied in a linear fashion in third trimester, while transverse cerebellar diameter/abdominal circumference (TCD/AC) ratio remained constant in second half of pregnancy. All the parameters were expressed by regression equations and correlation coefficients were found to be statistically significant (r=0.99 for TCD, r=0.98 for TCD/AC all p<0.0001).This study signifies that transverse cerebellar diameter and TCD/AC ratio on serial sonography give an accurate idea of gestational age. Cases with intrauterine growth restriction can be diagnosed if local, nomogram can be prepared for different ethnic groups.