Expected‐value bias in routine third‐trimester growth scans

ABSTRACT Objectives Operators performing fetal growth scans are usually aware of the gestational age of the pregnancy, which may lead to expected‐value bias when performing biometric measurements. We aimed to evaluate the incidence of expected‐value bias in routine fetal growth scans and assess its impact on standard biometric measurements. Methods We collected prospectively full‐length video recordings of routine ultrasound growth scans coupled with operator eye tracking. Expected value was defined as the gestational age at the time of the scan, based on the estimated due date that was established at the dating scan. Expected‐value bias was defined as occurring when the operator looked at the measurement box on the screen during the process of caliper adjustment before saving a measurement. We studied the three standard biometric planes on which measurements of head circumference (HC), abdominal circumference (AC) and femur length (FL) are obtained. We evaluated the incidence of expected‐value bias and quantified the impact of biased measurements. Results We analyzed 272 third‐trimester growth scans, performed by 16 operators, during which a total of 1409 measurements (354 HC, 703 AC and 352 FL; including repeat measurements) were obtained. Expected‐value bias occurred in 91.4% of the saved standard biometric plane measurements (85.0% for HC, 92.9% for AC and 94.9% for FL). The operators were more likely to adjust the measurements towards the expected value than away from it (47.7% vs 19.7% of measurements; P