The impact of choice of reference charts and equations on the assessment of fetal biometry.

OBJECTIVES

The assessment of fetal biometry is usually based on the comparison of measured values with predicted values derived from reference charts or equations in a normal population. This study was undertaken to assess the impact of the choice of reference charts and to develop a Z-score-based tool that could help sonographers to choose the reference charts that best fit their practice.

METHODS

Fetal biparietal diameter, head circumference, abdominal circumference and femur diaphysis length measurements were made at 20-24 and 30-34 weeks' gestation by four experienced sonographers. All measurements were transformed into Z-scores calculated according to three prediction equations (Snijders and Nicolaides, 1994; Chitty et al., 1994 and Kurmanavicius et al., 1999). Distributions of Z-scores were compared to the expected standard normal distribution based on mean, SD and Kolmogorov-Smirnov test. Simulations were made to assess sensitivity (Se), specificity (Sp) and Youden's index (Se + Sp - 1) of each reference equation, reflecting their ability to identify fetuses with abnormal biometry in our population. The reference that best fitted our practice was determined based on these results.

RESULTS

The Z-scores of all biometric parameters were significantly different (P < 0.001) when using any of the three reference equations, and none of the Z-score distributions could be considered similar to the standard normal distribution. The number of measurements that would be considered as abnormal according to these references ranged from 2.6% to 23.6%. Se and Sp ranged from 39.59% to 67.12% and 90.14% to 99.69%, respectively.

CONCLUSION

Assessment of fetal biometry is largely dependent on the choice of reference charts. We suggest that the choice of reference charts for fetal biometry could be controlled using Z-scores in each institution and that this could be the first step towards any quality assessment policy. The method we describe for the choice of the most appropriate fetal biometry reference chart might be used for all size charts.