Sonographic examination does not predict twin growth discordance accurately

Objective: To assess the accuracy of estimating birth weight among twins with discordancy (intra-pair difference in actual birth weight of more than 25%) and to determine the relative accuracy of an intra-pair difference in abdominal circumference (Δ AC) of 20 mm or more or in estimated fetal weight (Δ EFW) of 25% or more for the identification of discordant growth in twins. Methods: Over a 6-year period, we identified all non-anomalous twin pairs with gestational ages greater than 23 weeks and sonographic examinations within 3 weeks of birth. Ultrasonographic biometry of both twins included AC, head circumference, and femur length; these indices were used to estimate fetal weight by Hadlock's formula. Likelihood ratios, receiver-operating characteristic curves, and prediction limits were applied to assess the accuracy of the two diagnostic methods to predict an abnormal outcome. Results: A total of 242 twin pairs were studied. The mean gestational age among the 21 twins with abnormal growth (30.6 ± 4.6 weeks) was significantly less than among the 221 twins with normal growth (33.2 ± 4.0 weeks) (P < .005). The biometric measurements of fetal parts, sonographic estimate of fetal weight, and actual birth weight for both fetuses were significantly less for discordant twin pairs (P < .05). The accuracy of predicting birth weight, as determined by mean error and percentage of the estimate within 10% of the actual weight, was similar between the groups. Receiver-operating characteristic curves showed that both diagnostic tests yielded areas under the two curves not significantly different from the area under the nondiagnostic line (P > .05). Most important, prediction limit calculations indicated that a 90% certainty that the actual birth weight discordance was at least 25% was achievable only if Δ AC was 172 mm or greater or Δ EFW was 112% or more. Conclusion: The most popular current methods (difference in AC or EFW) for predicting discordant growth in twin gestations have limited accuracy when held to a standard for discordance that requires a birth weight difference of at least 25%.