Detection of small- and large-for-gestational age using different combinations of prenatal and postnatal charts

Objective: To determine the extent to which the detection rate of small-for-gestational age (SGA) and large-for-gestational age (LGA) at birth is influenced by the use of different combinations of estimated-fetal-weight (EFW) and birth-weight (BW) charts.

Methods: This was a cohort study of all pregnant women with a singleton term birth receiving care in a university hospital during a 3-year period. All participants underwent a universal 36-week ultrasound scan for EFW measurement and had BW recorded at delivery. Five different reference charts were used for EFW and BW centile calculation. Two-by-two contingency tables were constructed using EFW as the screening test variable and BW as the outcome variable in order to calculate sensitivity, specificity, positive predictive value (PPV) and negative predictive value for all possible chart combinations.

Results: The cohort included 17 678 pregnancies. The sensitivity of EFW < 10^th centile for the detection of BW < 10^th centile ranged from 10.8% to 66.8% and the sensitivity of EFW < 3^rd centile for the detection of BW < 3^rd centile ranged from 4.1% to 66.8%, depending on the charts used. The sensitivity of EFW > 90^th centile for BW > 90^th centile ranged between 22.9% and 68.3%. When locally derived charts for EFW and BW were used, the sensitivity of detection of BW < 10^th centile using EFW < 10^th centile was 43.7% (PPV, 45.5%); for the detection of BW < 3^rd centile using EFW < 3^rd centile, the sensitivity was 25.6% (PPV, 26.7%) and, for the detection of BW > 90^th centile using EFW > 90^th centile, it was 49.6% (PPV, 49.0%).

Conclusions: Different combinations of EFW and BW charts can yield vastly different detection rates (sensitivity) in the same population cohort and time period. If SGA and LGA detection rates are to be used as a meaningful performance indicator, healthcare systems should follow a clear and predefined methodology that includes explicit definitions of common reference standards. © 2022 International Society of Ultrasound in Obstetrics and Gynecology.