Objective: The purpose of this study was to assess the efficacy of routine ultrasonographic assessment of fetal weight in predicting fetal macrosomia in postdates pregnancies.
Study design: A total of 519 pregnancies of > or = 41 weeks' duration were subjected to ultrasonographic estimation of fetal weight within 1 week of delivery. Estimated fetal weights were compared with birth weights. Linear regression analysis was performed and prediction limits for estimated fetal weights were generated.
Results: Twenty-three percent of infants had birth weights > or = 4000 gm and 4% had birth weights > or = 4500 gm. The mean percent absolute error was 7.7%. At a birth weight of > 3750 gm, the Hadlock model (which uses abdominal circumference and femur length) systematically overestimated the birth weight. The sensitivity, specificity, and positive and negative predictive values for the ultrasonographic diagnosis of macrosomia were 56%, 91%, 64%, and 87%, respectively. The prediction limits generated determine the range of birth weights predicted for a given estimated fetal weight.
Conclusion: Routine ultrasonographic screening for macrosomia in postdates pregnancies is associated with a relatively low positive predictive value.