To assess intra‐ and interobserver variability of fetal biometry measurements throughout pregnancy.

A total of 175 scans (of 140 fetuses) were prospectively performed at 14–41 weeks of gestation ensuring an even distribution throughout gestation. From among three experienced sonographers, a pair of observers independently acquired a duplicate set of seven standard measurements for each fetus. Differences between and within observers were expressed in measurement units (mm), as a percentage of fetal dimensions and as gestational age‐specific Z‐scores. For all comparisons, Bland–Altman plots were used to quantify limits of agreement.

When using measurement units (mm) to express differences, both intra‐ and interobserver variability increased with gestational age. However, when measurement of variability took into account the increasing fetal size and was expressed as a percentage or Z‐score, it remained constant throughout gestation. When expressed as a percentage or Z‐score, the 95% limits of agreement for intraobserver difference for head circumference (HC) were ± 3.0% or 0.67; they were ± 5.3% or 0.90 and ± 6.6% or 0.94 for abdominal circumference (AC) and femur length (FL), respectively. The corresponding values for interobserver differences were ± 4.9% or 0.99 for HC, ± 8.8% or 1.35 for AC and ± 11.1% or 1.43 for FL.

Although intra‐ and interobserver variability increases with advancing gestation when expressed in millimeters, both are constant as a percentage of the fetal dimensions or when reported as a Z‐score. Thus, measurement variability should be considered when interpreting fetal growth rates. Copyright © 2012 ISUOG. Published by John Wiley & Sons, Ltd.