Background: Obesity is one of the most serious public health challenges of the 21^st century; maternal obesity represents a challenge in the sonographic (US) assessment of fetal weight, and is a recognized risk factor for adverse pregnancy outcome. Objective: To detect the possible effect of maternal obesity on the accuracy of sonographically estimated fetal weight in the third-trimester maximum seven days before labor and to compare the accuracy of the estimation between normal weight, overweight, and class I, class II and class III obese groups. Patient and Methods: This was a prospective study of 150 singleton pregnancies with sonographic fetal weight estimation prior to scheduled delivery. Women were classified according to current body mass index (BMI) into five categories: normal (BMI 18.5–24.9 kg/m², n = 30), overweight (BMI 25.0–29.9 kg/m², n = 30), obese class I (BMI 30.0–34.9kg/m², n = 30), obese class II (BMI, 35.0–39.9 kg/m², n = 30) and obese class III (BMI ≥ 40.0 kg/m², n = 30).The estimated fetal weight was compared with the actual birth weight, and the difference between them was recorded as the error. Results: There were statistically significant differences between US estimated fetal weight (EFW) and birth weight (g) versus body mass index in obesity class II and III. Conclusions: Maternal obesity decreased the accuracy of sonographic fetal weight estimation. Clinicians should be aware of the limitations of sonographic fetal weight estimation, especially in obese patients.