Gestational age determination by ultrasonography is very much essential as it gives accurate estimation. The objective of prsent work is to study the role of ultrasonography for estimation of gestational age and to assess of gestational age by ultrasonographic measurement of femur length. The study was carried out in the Department of Anatomy and in the Department of Radiodiagnosis at R.C.S.M.Government Medical College, Kolhapur, Maharashtra. This prospective cross sectional study consists of 150 normal antenatal women. The fetuses of singleton pregnancy with the physical evidence of normal growth without gross structural abnormalities with the gestational age ranging from18-40 weeks were included. The fetal femur length was measured by ulrasonography. The mean values of femur length in cm. along with respective standered deviation (sd) were computed for each gestaional age from 18-40 weeks.The correlation and regression analysis has been carried out to quantify the relationship between gestational age in weeks and femur length in cm. The femur length goes on increasing from 18 weeks to 40 weeks. The diphyseal length of femur is directly related to the gestational age. The simple linear regression equation derived for the calculation of gestational age using diphyseal length of femur. The relationship between the diphyseal length of femur and the gestaional age is linear and direct.Measurements of diphyseal lengths of femur can be an important parameter for estimating gestational age from 18-40 weeks of gestation.