Objective: To compare correct assessment of gestational age betweenTranscerebeller diameter versus femur length in third trimester (28-40) using first day of lastmenstrual period for actual period of gestation. Study Design: Cross-sectional descriptive study.Place and Duration of Study: Department of Obstetrics and Gynecology, Bahawal VictoriaHospital, Bahawalpur from Jun 2012 to Dec 2012. Methodology: This study was performed on327 patients in third trimester of pregnancy from 28-40 weeks fulfilling the inclusion criteria.Ultrasound measurements of transcerebellar diameter (TCD) and femur (FL) were made withcommercially available real time ultrasound equipment Toshiba Nemio-10 model 2009,Transducer frequency 50/60 Hz. Collected data was analyzed by SPSS version 10. Results: Outof 327 patients, TCD was found to give correct assessment corresponding to the gestational ageby LMP in 262 (80.1%) patients, while in 232 (70.9%) patients FL was found to give correctassessment corresponding to the gestational age by LMP. Conclusions: Transcerebellardiameter is more reliable method of gestational age determination in third trimester of pregnancythan femur length. TCD can be used as a tool to assist in the assessment of gestational age inthird trimester.