When delivery of a preterm infant is anticipated accurate antenatal assessment of the fetal weight can be a useful adjuvant for establishing a plan of management .That will minimize perinatal morbidity and mortality rate. Objective: To estimate fetal birth weight by measuring fetal abdominal circumference and biparietal diameter in 3rd trimester high risk pregnancies by Ultrasonography. To devise a simple and accurate equation for estimating birth weights of preterm and low birth weight fetuses. Methods and material: The study population was made up of 92 high risk 3rd trimester pregnant patients. Fetal measurements were made by real time USG machine. Result: In the present study estimated fetal birth weight calculated by Gray Thurnau’s equation formula was highly correlated with actual birth weight with correlation coefficient of 0.925. Conclusion: Abdominal circumference and biparietal diameter are the best indicators to assess birth weight in high risk pregnancies.