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Objective: the use of twin-specific versus singleton charts in the assessment of twin pregnancies has been controversial. The aim of the study was to assess whether a diagnosis of small for gestational age (SGA) made using twin specific estimated fetal weight (EFW) and birthweight (BW) charts is more strongly associated with adverse neonatal outcomes compared to singleton charts in twin pregnancies. Methods: this was a cohort study of twin pregnancies delivered at St George's Hospital in London between January 2007 and May 2020. Twin pregnancies complicated by intrauterine demise of one or both twins; aneuploidy or major fetal abnormality, twin-to-twin transfusion syndrome or twin anemia polycythemia sequence (TAPS); and those delivered before 32 weeks' gestation, were excluded. SGA was defined as EFW or BW below the 10th centile. The main study outcome was composite neonatal morbidity, which was stratified to mild or severe for sensitivity analysis. Mixed-effects logistic regression analysis with random pregnancy level intercepts was used to test the association between different SGA classifications and adverse neonatal outcomes. Results: a total of 1329 twin pregnancies were identified, and 913 twin pregnancies (1826 infants) included in the analysis. Of these, 723 (79.2%) were dichorionic and 190 (20.8%) monochorionic. Using the singleton charts, 33.3% and 35.7% were classified as SGA by the singleton chart when using EFW and BW, respectively. The corresponding figures were 5.9% and 5.8% when using the twin specific charts. EFW SGA according to the twin charts, had a significant association with neonatal morbidity (OR 4.78, 95% CI 1.47-14.7, P=0.007), when compared to AGA twins. However, EFW below the 10th percentile according to singleton standards did not have a significant association with neonatal morbidity (OR 1.36, 95% CI 0.63-2.88, P=0.424). SGA classification of EFW using twin specific standards significantly better model fit than using singleton standard, NA