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OBJECTIVE:The purpose of this study was to determine whether first trimester ultrasound scanning can identify twin gestations that are at risk for subsequent growth discordance. STUDY DESIGN/METHODS:Ultrasound examinations of dichorionic twin pregnancies between 11 and 14 weeks of gestation were evaluated for growth discordance with crown-rump length. Pregnancies that were complicated by fetal death or anomalies were excluded from the analysis. Birth weight discordance was defined as >or=20% difference in birth weight, relative to the larger twin. Fisher's exact, Mann Whitney U, and Spearman rho tests were used for statistical analysis. RESULTS:Of 130 twin pregnancies, 16 pregnancies (12.3%) had discordant birth weight. Crown-rump length disparity was correlated positively with birth weight discordance (r=0.31; P<.001). Of pregnancies with a discrepancy of <or=3 days in estimated gestational age, only 9.2% were discordant at birth compared with 45.5% of pregnancies with >3 days discrepancy (P=.004), which resulted in a likelihood ratio of 5.9 for having discordant birth weight. CONCLUSION/CONCLUSIONS:Twins who are ultimately discordant at birth may exhibit differences in growth as early as 11 to 14 weeks of gestation.