Longitudinal z Score Distribution in Sonographic Fetal Biometry: Influence of Examiner and Experience

To analyze the influence of examiners and their experience on the quality of biometric measurements via the evolution of z scores in a longitudinal multicenter study.

This retrospective study included 4607 sonographic fetal biometric examinations performed by 18 examiners at the beginning of their sonography training. To analyze the quality of biometric measurements, z scores comparing the individual measurements with the expected values from a standard population were computed. To investigate the effect of examiners and their experience, we applied a multivariable regression analysis via generalized additive mixed models.

Mean z scores for biparietal diameter, abdominal circumference, and femur length were statistically different from the expected value of 0 (P < .001); no significant differences were found for head circumference measurements. Regression analyses showed a significant effect of the number of examinations on the distribution of z scores for each type of measurement. This effect yielded z score values close to the expected value of 0 at 100 to 200 examinations for all biometric measurements, indicating good consistency with the distribution of values in the reference population. Near the end of the study period, an increasing tendency toward either overestimation (head circumference) or underestimation (biparietal diameter, abdominal circumference, and femur length) was observed.

A longitudinal analysis of the z score distribution for quality control of biometry is feasible. A prospective and automatized use of this technique could help identify potential systematic errors and therefore improve the detection rate for high-risk pregnancies.