Background: Determination of gestational age (GA) by biparietal diameter (BPD) on Ultrasonography is an important tool, especially in our part of world, where last menstrual period (LMP) of most of the patients is unknown. Aims and objectives: This study was carried out to assess gestational age by ultrasonography on the basis of BPD in comparison to GA by LMP. Materials and methods: A total of 99 normal pregnant females were studied with known last menstrual period. GA was determined by fetal BPD measurement in 2nd and 3rd trimester of pregnancy, with real time ultrasonography. GA by LMP and GA by BPD measurement were statistically correlated using paired t–test and Bland and Altman graph analysis. Observation and results: In this study GA in weeks by LMP was compared with GA in weeks by BPD. It was observed there is strong similarity in GA by LMP and GA by BPD in second trimester up to 29 weeks of gestation. It was determined by comparison of the gestational age by LMP and by BPD in study patients with p value 0.2433 and by difference between gestational age by the two methods stratified by gestational age by LMP [≥30 weeks had p value 0.112 and > 30 weeks had p value 0.0003]. Conclusion: Bi–parietal diameter is one of the useful criteria to measure GA and to predict expected date of delivery, especially up to 29 weeks of gestation.