Objective To test the hypothesis that the fetal fraction in twin pregnancy would reflect birthweight discrepancy. Design Retrospective cohort study. Setting International Peace Maternity and Child Health Hospital in Shanghai, China Population We included 237 twin pregnancies undergoing cfDNA screening for aneuploidy and delivered at International Peace Maternity and Child Health Hospital in Shanghai, China between January 2018 to December 2021. Exclusion criteria including abnormal NIPT results， fetal demise or structural abnormalities. Methods All women with twin pregnancies were offered a scan to determine chorionicity as well as first-trimester nuchal translucency (NT) at 11 ⁺⁰-13 ⁺⁶ weeks. Dichorionic was confirmed when ultrasound assessment clearly indicates two placentas. The twin peak sign is used to distinguish chorionicity if only one placenta is visualized. Main Outcome Measures Fetal fraction and birth weight of the new borns were collected. Relationships between fetal fraction and birth weight discordance or sFGR were analysed. Results Fetal fraction was positively correlated with the difference of birth weight (β=0.004, 95%CI: 0.001~0.006). Higher fetal fraction was significantly associated with the increased risk of birthweight discordance of 20% (adjusted OR:1.073, 95%CI: 1.009~1.142) and 25% (adjusted OR:1.092, 95%CI: 1.006~1.185) and sIUGR(adjusted OR:1.130, 95%CI: 1.038~1.231). We obtained the optimum cut-off point of fetal fraction ≥ 11.790, ≥ 14.800 and ≥ 14.800 for birthweight discordance of 20% and 25% and sFGR, respectively. Conclusion This study shown that fetal fraction was positively correlated with the difference of birth weight. Fetal fraction could be used as a biomarker in predating birth weight discordance and sFGR, and help to make individualized clinical monitoring plans for twin pregnancies.