Accuracy and modifying factors for ultrasonographic determination of fetal weight at term

To determine the accuracy of various sonographic formulas for estimating fetal weight at labor.

We evaluated prospectively four published equations by Hadlock et al, Shepard et al, Rose and McCallum, and Sabbagha et al, as well as clinical examination for accuracy in determining fetal weight during labor. Two hundred twenty-three 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.

The Hadlock and Shepard equations both had a lower percentage of error than the Sabbagha formula (6.1%/204 g and 6.2%/200 g respectively, versus 7.8%/271 g; P < .007). For all four equations, 70-79% of the fetal weight predictions were within 10% of actual birth weight. Sensitivities for detecting 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 did not affect predictive accuracy. The error in weight estimation varied between 6.3-8.1% in patients with oligohydramnios. Biparietal diameter (BPD) could not be measured in approximately two-thirds of the patients studied.

Using any of the four standard equations or clinical examination, accurate estimation of fetal weight can be achieved for patients in labor, even in the presence of ruptured membranes. Since the Hadlock equation does not rely on BPD measurements, it appears to be both the most accurate and clinically useful method for predicting fetal weight for patients in labor at term.