Estimating Gestational Age With Sonography: Regression‐Derived Formula Versus the Fetal Biometric Average

Objectives To compare the accuracy of a new regression‐derived formula developed from the National Fetal Growth Studies data to the common alternative method that uses the average of the gestational ages (GAs) calculated for each fetal biometric measurement (biparietal diameter, head circumference, abdominal circumference, and femur length). Methods This retrospective cross‐sectional study identified nonanomalous singleton pregnancies that had a crown‐rump length plus at least 1 additional sonographic examination with complete fetal biometric measurements. With the use of the crown‐rump length to establish the referent estimated date of delivery, each method's (National Institute of Child Health and Human Development regression versus Hadlock average [Radiology 1984; 152:497–501]), error at every examination was computed. Error, defined as the difference between the crown‐rump length–derived GA and each method's predicted GA (weeks), was compared in 3 GA intervals: 1 (14 weeks–20 weeks 6 days), 2 (21 weeks–28 weeks 6 days), and 3 (≥29 weeks). In addition, the proportion of each method's examinations that had errors outside prespecified (±) day ranges was computed by using odds ratios. Results A total of 16,904 sonograms were identified. The overall and prespecified GA range subset mean errors were significantly smaller for the regression compared to the average (P