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BACKGROUND Gestational diabetes (GDM) is associated with accelerated fetal growth in singleton pregnancies, but may impact twin pregnancies differently due to the slower growth of twin fetuses during the third trimester and their greater predisposition to fetal growth restriction. OBJECTIVE To evaluate the association of GDM with longitudinal fetal growth in twin pregnancies, and to compare this association with that observed in singleton pregnancies STUDY DESIGN: This was a retrospective cohort study of all women with a singleton or twin pregnancy who were followed at single tertiary referral center between Jan-2011 and Apr-2020. Data on estimated fetal weight and individual fetal biometric indices were extracted from ultrasound exams of eligible women. Generalized linear models were used to model and compare the change in fetal weight and individual biometric indices as a function of gestational age between women with versus without GDM in twin pregnancies, and, separately, for women with versus without GDM in singleton pregnancies. The primary outcome was estimated fetal weight as a function of gestational age. Secondary outcomes were longitudinal growth of individual fetal biometric indices and the rate of small for gestational age and large for gestational age at birth. RESULTS A total of 26,651 women (94,437 ultrasound exams) were included in the analysis: 1,881 with a twin pregnancy and 24,770 with a singleton pregnancy. The rate of GDM in the twin and singleton groups was 9.6% (n=180) and 7.6% (n=1,893), respectively. In singletons, estimated fetal weight in GDM pregnancies was significantly higher than in pregnancies without GDM (p<0.001) starting at approximately 30 weeks. Differences remained similar after adjustment for maternal age, chronic hypertension, nulliparity, and neonatal sex (p<0.001). In twin pregnancies, fetal growth was similar between pregnancies with and without GDM (p=0.105 and p=0.483 for unadjusted and adjusted models, respectively). Findings were similar with respect to the association of GDM with the risk of large for gestational fetuses and the growth of each individual biometric index. When stratified by type of GDM treatment, GDM in twins was associated with accelerated fetal growth only in the subgroup of women with medically-treated GDM (p<0.001), which represented 12% (n=21) of the Twin GDM group. CONCLUSION In contrast to singleton pregnancies, GDM in twins is less likely to be associated with accelerated fetal growth. This finding raises the question of whether the diagnostic criteria for GDM and the blood glucose targets in women diagnosed with GDM should be individualized for twin pregnancies.