OP20.09: The intra‐ and inter‐ observer variability in fetal measurements by ultrasound

Abstract

To establish intra- and inter- observer variability of fetal biometry measurements throughout pregnancy. One hundred and seventy five fetuses were prospectively recruited from the Oxford arm of the INTERGROWTH-21st cohort at 14–42 weeks of gestation ensuring an even distribution throughout gestation. From a pool of three experienced sonographers, a pair of observers independently acquired a duplicate set of 7 standard fetal measurements for each fetus. Differences between and within observers were expressed in actual measurement units (cm), as a percentage of fetal dimension, and as gestational age specific z score differences. Bland-Altman plots were used to quantify limits of agreement. When using differences in measurement units (cm) both the intra- and inter- observer variability increased with gestational age. However, when variability takes into account the increasing fetal size (expressed as a percentage of the measure or z score) it remains constant throughout gestation. The 95% limits of agreement for the intra-observer difference expressed as a percentage (or z score) are 3.2% (0.72) for head circumference (HC), 5.4% (0.81) for abdominal circumference (AC) and 6.8% (1.01) for femur length (FL), respectively. The corresponding values for the inter-observer differences were 5.1% (1.00) for HC, 8.9% (1.37) for AC and 12.0% (1.60) for FL. Although intra- and inter- observer variability increases with gestation when expressed in centimeters, both are constant as a percentage of the fetal dimensions or as a z score. Measurement variability should be considered when interpreting fetal growth rates.