Sonographic fetal weight estimation in prolonged pregnancy: comparative study of two- and three-dimensional methods.

OBJECTIVES

To compare two-dimensional (2D) and three-dimensional (3D) ultrasound techniques, including volumetry of fetal thigh, for fetal weight (FW) estimation in prolonged pregnancy, and to develop a new FW estimation formula.

METHODS

This prospective comparative study initially included 176 pregnant women. FW estimation was performed at >or= 287 days of gestation within <or= 4 days of delivery. Fetal head, abdomen and femur were measured using 2D ultrasound techniques, and fetal thigh volume was estimated using 3D techniques. The formula of Persson and Weldner (2D) was compared with two 3D formulae published by Lee and colleagues. In a subgroup of 63 fetuses, volumetry of the abdomen was performed and a new formula was developed; this formula was tested prospectively, along with the previously published formulae, on a further 50 women (Test Group).

RESULTS

In the initial group of 176 pregnancies, the SD of the mean percentage error (MPE) was 6.3% for both the 2D Persson and Weldner formula and for the better performing 3D formula of Lee et al., but the MPE of this Lee formula differed significantly from zero. Significantly more FW estimations were within +/- 10% of the birth weight when the 2D formula was used than when the 3D formulae were applied. The new formula gave a SD of MPE of 5.6% when applied to the data from which it was derived. In the Test Group, the SD of MPE was similar for the 2D formula, the second formula of Lee et al. and the new formula, with values of 7.0, 7.0 and 7.1, respectively, but only the Persson and Weldner formula showed a MPE that did not differ significantly from zero.

CONCLUSIONS

FW in prolonged pregnancies can be estimated using 2D sonography with the same accuracy as with 3D sonography. 3D ultrasound techniques require technically advanced and expensive equipment, special operator training and skills, and are time consuming. It does not seem reasonable to abandon the 2D ultrasound methods in favor of 3D ultrasound imaging for FW estimation.