Background: Obesity is one of the most serious public health challenges of the 21st century. Obesity has reached epidemic proportions worldwide. American College of Obstetricians and Gynecologists committee opinion, estimated that at least one- third of pregnant women are obese, and 8% are extremely obese. Objective: To detect the possible effect of maternal obesity on the accuracy of ultrasound fetal weight estimation during the third trimester shortly before labor. Patients and Methods: A prospective, comparative study was conducted at Om El Masryeen Hospital from August 2019 to August 2020. One hundred and fifty (150) singleton pregnant women who fulfilled the inclusion criteria were included in the study. All pregnant participants were between 37 and 42 weeks of gestation with a singleton cephalic presentation, and none of the participants had any medical or obstetrical problems. In the present work, women were classified into five BMI categories based on their current BMI each group included 30 patients. The study population was drawn from consecutive patients who underwent sonographic fetal weight estimation within 7 days of delivery and actual birth weight within thirty minutes after delivery. Results: By comparison between groups, we found that the mean of age was 27.4 ± 6.03 years, 27.8 ± 4.9 years, 28.8 ± 4.3 years, 28.6 ± 5.2 years and 28.6 ± 4.88 years in the normal weight, overweight, class I, class II and class III groups, respectively. Estimated fetal weight by ultrasound was 2.89±0.30kg, 3.00± .28kg, 3.21 ± .35kg, 2.99 ± 0.39kg and 3.31 ± 0.55kg in the normal weight, overweight, class I, class II and class III groups, respectively. Regarding actual birth weight was 2.95 ± 0.35kg, 3.09 ± 0.30kg, 3.23±0.34kg, 2.92 ± 0.45kg and 3.13±0.55kg in the normal weight, overweight, class I, class II and class III groups, respectively. Statistically significant difference between ultrasound (US) estimated fetal weight (EFW) and birth weight versus body mass index in obesity class II and III. Conclusion: 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.