Sonographic accuracy of estimated fetal weight in twins

Objectives - We aimed to determine the accuracy of sonographic formulas for estimating birth weight in twin pregnancies. Methods - We conducted a retrospective cohort study of consecutive twin pregnancies undergoing sonography at within 1 week of delivery. Pregnancies were included if biometric measurements and birth weight were available and excluded if anomalies were present. Estimated fetal weight was calculated using three sonographic formulas: two derived from singletons (Hadlock and Shepard) and one from twins (Ong). The correlation between estimated fetal weight and birth weight was determined using the Pearson correlation coefficient. The accuracy of each formula (bias) was assessed using the mean percentage error [(estimated fetal weight - birth weight)/birth weight × 100], and the precision (random error) was estimated from the standard deviation of the percentage error. The screening efficiency of each formula for intrauterine growth restriction, defined as below the 10th percentile on the Alexander growth standard, was assessed. The effect of twin presentation was determined using a paired analysis. Results - Of 1744 consecutive twin pregnancies, 270 (540 infants) met inclusion criteria. The estimated fetal weight of all 3 formulas strongly correlated with the birth weight (Pearson r = 0.90 for Hadlock, 0.87 for Shepard, and 0.92 for Ong). Each formula had similar sensitivity (65%-70%) and specificity (85%-90%) for intrauterine growth restriction. For each formula, the correlation coefficient was similar between twins A and B (Pearson r = 0.85-0.93); however, the estimated fetal weight for twin A tended to underestimate birth weight, whereas the estimated fetal weight for twin B tended to overestimate birth weight. Conclusions - Three widely used estimated fetal weight formulas, two derived from singletons and one from twins, perform equally well in estimating birth weight in twin gestations.