Is sonographic assessment of fetal weight influenced by formula selection?
OBJECTIVE
Several published formulas exist for the determination of estimated fetal weight (EFW), with limited data on their comparative accuracies. The aims of our study were to assess and compare the performance of different EFW formulas in predicting actual birth weight (BW) in an urban population.


METHODS
Patients with an EFW determined within 7 days of delivery were considered eligible for the study. Fourteen published formulas, derived from populations comparable to ours, were used to recalculate EFWs from the same initial measurements. The accuracy of the EFWs obtained from the different formulas were compared by percentage error methods using bias and precision and Bland-Altman limits of agreement methods. Sensitivity and specificity for prediction of being small for gestational age (SGA) were calculated.


RESULTS
Eighty-one fetuses were included in the study. Formula C of Hadlock et al [Hadlock C; log(10) BW = 1.335 - 0.0034(abdominal circumference [AC])(femur length [FL]) + 0.0316(biparietal diameter) + 0.0457(AC) + 0.1623(FL); Am J Obstet Gynecol 1985; 151:333-337] had the best performance according to the bias and precision method. Bland-Altman limits of agreement confirmed these results. Among the formulas, the sensitivity for detection of SGA ranged from 72% to 100%, and specificity was 41% to 88%. Hadlock C had the optimal sensitivity/specificity trade-off for detection of SGA.


CONCLUSIONS
Fourteen formulas showed considerable variation of bias and precision in our population as well as a wide range of sensitivities and specificities for SGA. The choice of the appropriate formula for EFW in a given population should be based on objective and explicit criteria. Consideration of bias and precision for the formula in the population being assessed is critical and may affect clinical care.
Burd
Irina
I
Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, University of Pennsylvania, 2000 Courtyard, 3400 Spruce St, Philadelphia, PA 19104-6142 USA. irina.burd@uphs.upenn.edu
Srinivas
Sindhu
S
Paré
Emmanuelle
E
Dharan
Vanita
V
Wang
Eileen
E
D000465:Algorithms; D000886:Anthropometry / Q000379:methods*; D001724:Birth Weight / Q000502:physiology*; D020567:Fetal Weight; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D010414:Pennsylvania; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal / Q000379:methods*