OBJECTIVE: Gestational diabetes mellitus (GDM) causes both short- and long-term harm to mothers and fetuses. It is important to predict the occurrence of GDM as early as possible and take adequate measures to prevent it. The purpose of this study was to investigate the association between body composition of pregnant women in early pregnancy and the risk of GDM. SUBJECTS AND METHODS: A total of 1318 pregnant women in the early stage of pregnancy were recruited from the Guiyang Maternal and Child Health Care Hospital. Detailed clinical data were recorded. Body composition was determined using the bioimpedance method at 13 weeks of gestation. The association between BMI before pregnancy (pre-BMI), fat mass percentage (FMP) and skeletal muscle mass percentage (SMMP) and the results of glucose tolerance screening in the second trimester of pregnancy were analyzed. Fat mass index (FMI) was calculated using fat mass in kilograms as measured using BIA at 13 weeks of pregnancy divided by the square of the woman's height in meters (kg/m2) and was analyzed to determine the predictive effect of body fat on GDM. RESULTS: Of the 1318 participants, 249 were diagnosed with GDM and 1069 with normal blood glucose. The FMI and FMP in GDM were higher than in NGT (P<0.001), while the SMMP in GDM was lower than in NGT (P<0.001). Overweight women (pre-BMI≥24kg/m2) had a higher risk of developing GDM than women with normal pre-BMI (adjusted OR 2.604, 95% CI 1.846-3.673). Women with FMP greater than 28% had a higher risk of developing to GDM than women with normal-range FMP (adjusted OR 1.572, 95% CI 1.104-2.240). When FMI is used to predict the incidence of GDM, the area under the curve (AUC) is 65.8%, which is comparable to BMI (AUC=67.2%). CONCLUSION: Body composition early during gestation is associated with the risk of GDM. The fat mass index in early pregnancy is a predictor of GDM, and it could be an indicator of the efficacy of any intervention to reduce the risk of GDM.