686: Predictions of large for gestational age by various sonographic fetal weight estimation formulas--how accurate are we?


      To evaluate the accuracy of various formulas of sonographic estimation of fetal weight (sEFW) for detection of large for gestational age (LGA) neonates at term.

      Study Design

      Retrospective cohort study of singleton gestations, 37+0/7-42+0/7 weeks of gestation, with sonographic biometrical measurements taken up to 7 days prior to delivery and no chromosomal/ structural anomalies (2007-2014). SGA neonates were excluded. sEFW was calculated by twenty published formulas. LGA was defined as birth weight above the 90th percentile for gestational age by local reference values. The following indices were calculated: sensitivity, specificity, positive and negative predictive values (PPV and NPV), positive and negative likelihood ratios (+LR and-LR), area under the curve of a receiver operating characteristics curve (AUC-ROC). Additionally, systematic and random errors (SE and RE) and the proportion of estimates (POE) within 10% of actual birth weight were calculated in order to compare each formula’s performance among LAG and non-LAG neonates.


      Out of 62,102 deliveries, 7,381 met the inclusion criteria, of which 1,613 neonates were LGA (22%). There was a considerable variation in formulas sensitivity (74.6±16.3%, 23.5-99%), specificity (86.3±10.6%, 51.7-99.6%), PPV (64.9±12.4%, 35.6-93.8%), +LR (9.3±10.9, 2.1-54.2) and -LR (0.3±0.16, 0.02-0.8), a mild variation in NPV (92.9±3.7%, 82.3-99.5%) and a minimal variation in AUC-ROC (94.3%, 93.0-95.1). Overall, 8/20 (40%) formulas had lower absolute SE for LGA compared with AGA neonates, 19/20 formulas (95%) had lower RE for LGA compared with AGA, and 14/20 (70%) had higher rate if POE within 10% of actual birth weight for LGA compared with AGA neonates. The overall ranking of the formulas is presented in the table.


      There is a wide variation in EFW formulas performance for detecting LGA. Hadlock’s formula (1985) combining abdominal circumference, femur length and biparietal diameter ranked highest.
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