Ultrasound Measurement Variability Between Sonographers
Yes, a 1cm discrepancy in fetal length measurements between two ultrasounds performed 4 weeks apart can absolutely be due to different sonographers performing the scans, as inter-operator variability is a well-documented phenomenon in prenatal ultrasound. 1
Evidence for Sonographer-Specific Measurement Variation
Research trials have demonstrated that important differences in ultrasound measurements exist between sonographers, with studies showing that the use of a single master set of medians was associated with lower screening performance than sonographer-specific medians. 1 This directly confirms that different operators can produce systematically different measurements even when properly trained.
Guidelines from Genetics in Medicine explicitly acknowledge that "it may not be necessary (or possible) to achieve the uniform performance level required to allow use of only one universal set of medians" across all sonographers. 1
Quality assurance programs monitor individual sonographer performance using three epidemiological parameters, with acceptable median measurements ranging from 0.90 to 1.10 MoM (multiples of the median), indicating that a 10% variation between operators is considered within acceptable limits. 1
Additional Sources of Measurement Discrepancy
Beyond operator variability, several technical factors can contribute to measurement differences:
Fetal positioning and flexion: The actual measurement technique matters significantly—in cases of curved structures, measuring the actual length versus the linear distance between endpoints can yield different results. 2
Equipment differences: Inferior ultrasound equipment or different machines between facilities can produce measurement variations. 1
Measurement timing within growth intervals: Due to inherent variability and standard deviations in measurements, growth assessments should typically be performed no more frequently than every 2 weeks to allow meaningful interval change. 1
Clinical Context and Reassurance
For routine fetal biometry, measurement accuracy in the second and third trimesters is approximately ±10 days when using composite measurements. 3, 4 A 1cm discrepancy in a single measurement parameter over 4 weeks falls within expected measurement variability and does not necessarily indicate abnormal growth.
The International Society of Ultrasound in Obstetrics and Gynecology emphasizes that fetal growth is a dynamic process requiring at least two scans separated in time, and that interval growth assessment is more clinically meaningful than isolated measurements. 4
Serial scans should be used to determine if interval growth has been normal rather than recalculating gestational age with each scan. 4
Recommended Action
If there is concern about the measurement discrepancy, request that the same sonographer perform the next ultrasound examination, or ensure that measurements are interpreted using sonographer-specific or center-specific reference medians rather than universal standards. 1 This approach accounts for systematic differences between operators while maintaining appropriate clinical surveillance.