title: Sonographic examination does not predict twin growth discordance accurately.
abstract: 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 (delta AC) of 20 mm or more or in estimated fetal weight (delta 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 delta AC was 172 mm or greater or delta EFW was 112% or more.
  
  
  CONCLUSIONS
  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%.
authors.affiliation: Department of Obstetrics and Gynecology, Medical College of Georgia, Augusta 30912-3350, USA.
authors.forename: J W
authors.initials: JW
authors.lastname: Caravello
authors.forename: S P
authors.initials: SP
authors.lastname: Chauhan
authors.forename: J C
authors.initials: JC
authors.lastname: Morrison
authors.forename: E F
authors.initials: EF
authors.lastname: Magann
authors.forename: J N
authors.initials: JN
authors.lastname: Martin
authors.forename: L D
authors.initials: LD
authors.lastname: Devoe
mesh_terms: D001724:Birth Weight; D005314:Embryonic and Fetal Development; D005260:Female; D006801:Humans; D011237:Predictive Value of Tests; D011247:Pregnancy; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014427:Twins; D016216:Ultrasonography, Prenatal