Screening for fetal growth restriction: A mathematical model of the effect of time interval and ultrasound error

Objective: We estimated the effect of ultrasound error and time interval between examinations on the false-positive rate for detecting fetal growth restriction (FGR). Methods: Using published growth curves for the fetal abdominal circumference and a coefficient of variation for ultrasound error of 5%, computer simulation was used to estimate false-positive rates in relation to the time interval between ultrasound examinations. Growth restriction was diagnosed when there was no apparent growth in fetal abdominal circumference between two consecutive examinations. In separate studies, the false-positive rate was plotted against gestational age at the first ultrasound examination. Results: There was a dramatic increase in false-positive rates as the time interval between examinations was reduced. When the initial scan was performed at 32 weeks, the false-positive rate increased from 3.2% for an interval of 4 weeks to 30.8% for an interval of 1 week. At a a-week interval, the error was 16.9%. There was a significant increase in the false-positive rate as the gestational age at the initial ultrasound was increased. At 28 weeks, the false-positive rate with a 2-week interval was 11.8%, increasing to 24.1% at 38 weeks. By varying the coefficient of variation of the ultrasound error, the false-positive rate increased from 0.8% at an error of 2% to 31.9% at an error of 10%. Conclusion: Ultrasound scanning at 2-week intervals is associated with false-positive rates for growth restriction in excess of 10%, increasing to much higher rates late in the third trimester. Improved screening performance should be attainable by increasing the interval between scans and reducing measurement errors. (Obstet Gynecol 1998;92:908-12. (C) 1998 by The American College of Obstetricians and Gynecologists.).