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Bland–Altman plots of the interobserver differences in the measurement of biparietal diameter (BPD) (a), head circumference (HC) (b), abdominal circumference (AC) (c) and femur length (FL) (d), showing that the variation increased with the magnitude of the measurements for BPD, HC and AC. For each graph, the solid line represents the mean difference and the dashed lines are the mean difference ± 2 SD.
