ACCURACY AND MODIFYING FACTORS FOR ULTRASONOGRAPHIC DETERMINATION OF FETAL WEIGHT AT TERM

Objective: To determine the accuracy of various sonographic formulas f or estimating fetal weight at labor. Methods: We evaluated prospective ly four published equations by Hadlock et al, Shepard et al, Rose and McCallum, and Sabbagha et al, as well as clinical examination for accu racy in determining fetal weight during labor. Two hundred twenty-thre e patients at 35-42 weeks' gestation underwent sonographic examination . Amniotic fluid index (AFI), placental location, maternal weight, and fetal station were recorded for each study. Results: The Hadlock and Shepard equations both had a lower percentage of error than the Sabbag ha formula (6.1%204 g and 6.2%/200 gr respectively, versus 7.8%/271 g; P < .007). For all four equations, 70-79% of the fetal weight predict ions were within 10% of actual birth weight. Sensitivities for detecti ng birth weights greater than 3800 g varied greatly (11-76%), whereas specificities for detecting birth weights less than 3800 g exceeded 88 %. For most equations, AFI, placental location, and maternal weight di d not affect predictive accuracy. The error in weight estimation varie d between 6.3-8.1% in patients with oligohydramnios. Biparietal diamet er (BPD) could not be measured in approximately two-thirds of the pati ents studied. Conclusion: Using any of the four standard equations or clinical examination, accurate estimation of fetal weight can be achie ved for patients in labor, even in the presence of ruptured membranes. Since the Hadlock equation does not rely on BPD measurements, it appe ars to be both the most accurate and clinically useful method for pred icting fetal weight for patients in labor at term.