Ultrasonographic fetal weight estimation by an integrated computer-assisted system: can each laboratory improve its accuracy?

A systematic review of the ultrasound estimation of fetal weight

OBJECTIVES

The range and use of ultrasound fetal measurements have gradually been extended. Measurements have been combined to estimate fetal weight by mathematically based non-linear regression analysis or physically based volumetric methods. Fetal weight estimation is inaccurate, with poor sensitivity for prediction of fetal compromise. Several authors have shown the unacceptable level of intra- and interobserver variability in fetal measurement and the impact of errors on growth assessment. The aims of this study were to review the available methods and possible sources of inaccuracy.

METHODS

Four databases were searched for studies comparing ultrasound estimated fetal weight (EFW) with birth weight. Studies meeting the inclusion criteria evaluated 11 different methods. Errors were graphically summarized.

RESULTS

No consistently superior method has emerged. Volumetric methods provide some theoretical advantages. Random errors are large and must be reduced if clinical errors are to be avoided.

CONCLUSIONS

The accuracy of EFW is compromised by large intra- and interobserver variability. Efforts must be made to minimize this variability if EFW is to be clinically useful. This may be achieved through averaging of multiple measurements, improvements in image quality, uniform calibration of equipment, careful design and refinement of measurement methods, acknowledgment that there is a long learning curve, and regular audit of measurement quality. Further work to improve the universal validity and accuracy of fetal weight estimation formulae is also required.

Ultrasonic fetal weight estimation and tolerance to measurement error: A comparative analysis

The objective of the present study was to test the hypothesis that volumetric formulae for fetal weight estimation are more tolerant to measurement error than exponential models. A mathematical model of normal fetal growth was constructed, using published British reference standards for biometric variables. Observed measurements were computer-generated by contaminating reference measurements with error terms according to their published coefficients of variation. The error in weight estimation was computed as the percentage difference between weight estimates derived from the observed biometric variables and the true measurements. A total of nine weight estimation formulae were tested. Campbell's formula appeared to be most affected by observational errors, especially before 38 weeks. In this range, they varied up to 9%. The most tolerant was Shepard's formula, with errors of only approximately 2.8%. Other formulae showed errors of approximately 5-6%. With the exception of Campbell's formula, the effect of gestational age was minimal. There was no correlation between percentage error and fetal size. Combining ultrasound biometric variables into a fetal weight estimate does not always exaggerate the errors of the original measurements. There were no significant differences between volumetric and exponential formulae.

Prediction of fetal weight by ultrasound: the contribution of additional examiners

OBJECTIVES

To assess the contribution of additional examiners to: the average discrepancy between estimated and actual fetal weights; the correlation between estimated and actual fetal weights; the reduction in major (> 10%) discrepancies between estimated and actual fetal weights.

DESIGN

Three experienced sonographers independently measured fetal biparietal diameter, head circumference, abdominal circumference and femur length in 39 fetuses at term. The estimated fetal weights were calculated for each examiner. Fetal biometric measurements were analyzed to obtain the source of differences in estimations among the examiners. Discrepancy, correlation and number of major (> 10%) discrepancies between the estimated and actual fetal weights were calculated for each examiner, and the contribution of additional examiners was analyzed.

RESULTS

The differences in measurements of the biparietal diameter and femur length were lower than those of the head and abdominal circumferences. For each of the three examiners, the average discrepancy between the estimated and actual fetal weights was 6.1%, 5.9% and 6.3%. When the estimation was based on two examiners, the discrepancy decreased to 4.8-5.6%. The contribution of a third examiner was nil. Major (> 10%) discrepancies between estimated fetal weight and actual birth weight were found in seven, eight and nine estimations of the examiners. Estimation by two examiners decreased the number of major discrepancies, and estimation by all three examiners further decreased by approximately 50% the number of major discrepancies between the estimated and actual fetal weights.

CONCLUSION

Measurements by multiple examiners changes only slightly the average number of discrepancies between estimated and actual fetal weights. However, the reduction in major (> 10%) discrepancies is statistically and clinically significant.