Objective :To study whether fetal abdominal fat can predict fetal low birth weight in term pregnancy. Study design :Descriptive diagnostic test study Setting : Labor room, Srinagarind Hospital, Faculty of Medicine, Khon Kaen University. Subjects :Three hundred twenty eight term pregnant women with singleton pregnancies who were admitted for delivery at Sriganarind hospital between September 1, 2000 to July 30, 2001. Methods :Term pregnant women were examined by ultrasound. Fetal abdominal fat thickness was measured using the same plane as the abdominal circumference. Main outcome measures : Fetal abdominal fat thickness in millimeters and birth weight in grams. Results :Three hundred and five fetuses were normal birth weight , 10 were low birth weight (weight < 2,500 g), and 13 were macrosomia (weight > 4,000 g). The incidence of low birth weight in this study was 3.0% . The fetal abdominal fat thickness ranged between 2.8 - 6.3 mm in all fetuses, with a mean of 4.2 + 0.56 mm . There was a significant positive correlation between the abdominal fat thickness and the birth weight (r = 0.512 , p < 0.0005). Infant with abdominal fat thickness less than or equal 4 mm were more likely to have low birth weight. The diagnostic performance of this method was: a sensitivity of 90.0% (95%CI=86.8-93.3), a specificity of 53.5% (95% CI=48.1-58.9), a positive predictive value of 5.7% (95% CI=3.2-8.3) and negative predictive value of 99.4% (95% CI=98.6-100.0) Conclusion :Sonographic measurement of the fetal abdominal fat thickness less than or equal 4 mm may be useful for screening of low birth weight infant.