Sonographic estimation of fetal weight based on a model of fetal volume.

OBJECTIVES: To derive a formula for sonographic estimated fetal weight (EFW) based on a two-compartment model of fetal volume and to test it against two widely used formulas, especially at the extremes of fetal weight for which existing formulas are generally inaccurate.

METHODS: We analyzed 865 consecutive sonograms that met the following inclusion criteria: singleton pregnancy, normal anatomy, delivery within 3 days of sonography, and measurements of biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). The weight of the fetal head was modeled to be proportional to HC3, and the weight of the trunk proportional to AC2 x FL. The proportionality constants were found by multiple linear regression on 380 sonograms performed in 1990 (the "derivation set"). The new formula was tested for accuracy of prediction of actual birth weight against the formulas of Hadlock et al and Shepard et al using 485 sonograms from 1991-1992 (the "validation set").

RESULTS: In the derivation set, the formula EFW = (0.23718 x AC2 x FL) + (0.03312 x HC3) was fit; the correlation with actual birth weight had an r value of 0.996. In the validation set, the new formula produced smaller systematic errors and smaller absolute errors than either the Hadlock or Shepard formula both overall and in fetal weight strata from less than 1000 g to over 4000 g.

CONCLUSION: The new formula makes geometric sense and provides accurate estimates of fetal weight across a broad range of weights.