First-Trimester Abdominal Circumference (Versus Crown Rump Length) Improves Precision in Inter- and Intraobserver Variability

Abstract

Objectives: The aim of this study was to evaluate inter- and intraobserver variability of first-trimester biometric measurements and crown-rump length (CRL) and to compare the accuracy and precision of CRL with these biometric measurements used in the interpretation of first-trimester nuchal translucency (NT).

Methods: Women presenting for a first trimester ultrasound were recruited. Both a sonographer, and a maternal fetal medicine specialist individually examined each participant. Each examiner obtained three independent measurements of CRL, a standardized set of biometric measurements (biparietal diameter, occipitofrontal diameter, head circumference, abdominal circumference, humerus length, and femur length), and an NT between 11 0/7 and 13 6/7 weeks of gestation. Biometry-specific expected NT values were calculated using linear and quadratic regression models and were used to convert results into multiples of the median.

Results: Fetal biometric measurements, CRL, and NT measurements were obtained in 356 consecutive pregnancies with singleton fetuses. CRL demonstrated the least intra- and interobserver variability as demonstrated by the smallest coefficient of variance. However, abdominal circumference and head circumference were not statistically different from CRL variance. CRL and abdominal circumference showed the smallest standard deviation when calculating multiples of the median for NT interpretation.

Conclusion: First-trimester abdominal circumference demonstrates the most intra- and interobserver precision for dating and calculating NT multiples of the median, which could potentially be useful with obesity and in any setting with technical limitations of sonography.

Keywords: crown-rump length; fetal biometry; first trimester; nuchal translucency; prenatal screening; ultrasound.