Abstract: Objective To investigate characteristics of maternal body composition of pregnant women with gestational diabetes mellitus (GDM) in the last trimester. Methods A nested case-control study in 1 261 pregnant women from our multicenter cohort from May 20, 2012 to December 31, 2013 was conducted. There were 135 cases with GDM in GDM group and another 135 prepregnant body weight- and height-matched women with uncomplicated pregnancies in control group. Bioelectrical impedance analysis was used to determine body composition in subjects at 38-39 gestational weeks. The body mass (BM), fat mass (FM), fat mass percentage (FMP), free fat mass (FFM), muscular mass (MM), total body water (TBW), extracellular water (ECW), intracellular water (ICW), fetal birth mass (FBM) and fetal birth length (FBL) were analyzed and compared between the two groups. Results The BM (72.15±11.18) kg, FFM (48.41±5.36) Kg, MM (43.24±5.71) kg, TBW (33.87±4.40) kg, ECW (15.84±2.06) kg, ICW (17.95±2.67) kg and FBM (3 551.6±364.5) g in the last trimester in GDM group were significantly higher than control group which were BM (70.39±9.56) kg, FFM (47.22±4.75) kg, MM (41.64±4.97) kg, TBW (32.56±3.63) kg, ECW (15.42±1.82) kg, ICW (17.04±2.27) kg and FBM (3 397.6±417.7) g (P< 0.05), but there were no significant differences in FM, FMP and FBL between two groups (P> 0.05). Conclusion The body fat reserves of GDM pregnant women in the last trimester has no difference with normal pregnant women, higher BM and FBM in the GDM are attributed to higher maternal FM, MM and TBW in the last trimester.