HOW TO USE Z SCORES FOR THE QUALITY CONROL OF FETAL BIOMETRY. A FRENCH OVERVIEW FROM DATA COLLECTION TO Z SCORE DISTRIBUTIONS

Purpose: To construct new reference charts and equations for fetal biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL) using a large sample of fetuses examined at 15–40 weeks in France. To develop a Z-score-based tool to compare this new reference with previous ones and to develop and evaluate the feasibility of a Z score distribution-based quality assessment for fetal biometry.

Materials and methods: The study data were obtained from a population of pregnancies undergoing ultrasound examination over a continuous one year period. Exclusion criteria were: known abnormal karyotype or congenital malformation, multiple pregnancies, no first trimester dating based on crown-rump length No exclusion was made on the basis of abnormal biometry or birth weight. For each measurement, separate regression models were fitted to estimate both the mean and standard deviation at each gestational age. Predicted centiles were compared among references. Statistical means of analysing sonographer's Z score distribution were investigated.

Results: Complete biometric measurements were obtained in 19647 fetuses. New charts and reference equations are presented for BPD, HC, AC and FL. Prediction intervals for the new reference were comparable to those of previous references, whereas there were some difference in the predicted centiles among references. Distributions of Z scores were compared to the expected standard normal distribution based on mean, standard deviation (SD) and Kolmogorov-Smirnov test.

Conclusion: We have presented new French reference charts and equations for fetal biometry. They can be easily used to compute centiles and Z-scores, to control quality of biometric assessment as well as to evaluate their performance as compared to other references.