Background: Pre-eclampsia, gestational hypertension, macrosomia, induction of labor, and cesarean sections are all connected with higher risk for both lean and obese women. Aim of the Work: The aim of this study was to determine the effect of maternal body mass index (BMI) on the accuracy of sonographic estimated fetal weight (EFW) above 37 weeks' gestation. Methods: This is a prospective Cohort study and was conducted at Obstetrics and Gynecology department at Beni-Suef General hospital starting from July 2019 till February 2020, on 300 pregnant women. According to their body mass index (BMI) they categorized into 6 groups (27 underweight, 63 average, 102 overweight, 51 obese class I, 42 obese class II and 15 obese class III), Ultrasonographic estimation of fetal body weight was done using Hadlock 3 formula. Also, amniotic fluid index was measured by ultrasonography. Results: There was a statistically significant difference between the studied groups as regard fetal biometrics “BPD, HC, AC and FL", estimated fetal weight and actual birth weight between the six groups. There was no significant difference between the different groups of the study absolute error of change and mean percentage error of change between EFW and AFW. According to AFI, 21 (7.0%) of our participants were having oligohydramnios, 272 (90.7%) were having normal AFI and 7 (2.3%) were having polyhydramnios.Conclusion:  There was a statistically significant relationship between mother body mass index and infant birth weight. The correlation between BMI and estimated and actual birth weight was positive. There is a strong correlation between maternal obesity and having a baby that is big for its gestational age. Furthermore, extremes in amniotic fluid content may influence sonographic assessment of baby weight in late pregnancy.