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Maternal obesity represents a challenge in the sonographic (US) assessment of fetal weight, and is a recognized risk factor for adverse pregnancy outcome. The objective of this secondary analysis of data from the Prospective Observational Trial to Optimize Pediatric Health in fetal growth restriction (FGR) Study (PORTO) was to describe the effect of maternal obesity on the accuracy of US in determining the estimated fetal weight (EFW) and the perinatal outcome of pregnancies affected by FGR.Between 2010 and 2012, 1,116 women with nonanomalous singleton pregnancies with an EFW in less than the tenth centile were recruited for the PORTO study. Maternal body mass index (BMI) was divided into five subcategories: normal (BMI 24.9 kg/m(2) ), overweight (25-29.9), obese class 1 (30-34.9), obese class 2 (35-39.9), and obese class 3 (40). The accuracy of the EFW was determined in women who delivered within 2 weeks of their last US scan. Perinatal outcomes were analyzed by BMI subcategory.Of the 1,074 patients with complete records, 691 (64%) were of normal weight, 258 (24%) were overweight, 93 (9%) were in obese class 1, 32 (3%) were in obese class 2, and none were in obese class 3. Overall, the EFW determined prior to delivery was within 6% of the actual birth weight in all BMI subcategories. Overweight and obese women delivered more commonly by cesarean section and at earlier gestational ages than did women with a normal BMI (p = 0.0008), resulting in lower birth weights (p = 0.0031) and significantly increased composite perinatal morbidity (p 0.0001) and mortality (p = 0.0215) rates.US examination is reliable for assessing the weight of fetuses with FGR in overweight women. Maternal obesity, however, has a significant adverse effect on perinatal outcomes. Thus, health education should focus on awareness of this adverse effect, with optimization of prepregnancy weight as its main goal.