Background: The cerebellum, the largest part of hind brain, lies in the posterior cranial fossa. In the embryo cerebellum appears at the end of the fifth week as a swelling overriding the fourth ventricle. Assessment of gestational age (G.A.) is important in the management of pregnancy. The aim is to assess the reliability of trans-cerebellum diameter in compare with the femur length in predicting gestational age. Methodology: This is descriptive, cross-sectional study carried out on seventy-one pregnant women with history of reliable menstrual cycles (sure of dates, regular cycles, no history of hormonal contraception at last 6 months, no history of abortion or delivery at last 3 months) and did not suffer from any complications for the mother or fetus, referred for routine ultrasound examination in ultrasound department of El Minia University maternity hospital, FL was used as standard method to estimate gestational age to be compared with the other gestational age that obtained with TCD. Results: among pregnant women in the GA 28-32 and 32-36 weeks, there was non-statistically significant difference between GA as estimated by TCD and as estimated by other assessed fetal biometry (SFH and FL).  we found that TCD can correctly predict GA in early 3^rd trimester (28-36 weeks) at cut-off point >34.9 mm with a sensitivity of 94%. Conclusions: The result of the study revealed that is there is no significant difference between gestational age obtained with FL and TCD. The study recommended that the TCD can be used as routine measurement in estimation of the gestational age in the third trimester.
 
Keywords: Gestational Age, Femur Length, Trans-cerebellar Diameter, Third trimester.