Abstract: Objective To investigate characteristics of the maternal body composition in the second and last trimester of pregnant women delivered macrosomic fetus. Methods Nested case-control study design was adopted, and samples of pregnant women were from our multicenter cohort from May 20, 2012 to December 31, 2013.There were 62 cases who had macrosomic infants(≥ 4 000 g) delivery in the 38-41 gestational weeks in macrosomia group and another 62 age-, pre-pregnant body weight-and height-matched pregnant women who had a neonatal birth weighing 2 500 to 4 000 g served as control group.The maternal body composition in subjects at 14-16 gestational weeks and 36-38 gestational weeks were tested by multi-frequency bioelectrical impedance analysis.The body mass(BM), body mass index(BMI), free fat mass(FFM), muscular mass(MM), fat mass(FM), fat mass percentage(FMP), extracellular water(ECW), intracellular water(ICW) and total body water(TBW) were analyzed and compared between the two groups. Results The BM, ECW in the second trimester and BM, BMI, FFM, MM, ECW, ICW, TBW in the last trimester in macrosomia group were significantly higher than control group(64.80±9.52) kg vs(62.68±8.43) kg, (14.26±1.64) kg vs(13.64±1.83) kg; (74.86±9.38) kg vs(70.86±8.39) kg, (27.82±3.13) kg vs(26.71±2.80) kg, (50.07±5.91) kg vs(47.27±4.50) kg, (44.59±6.21) kg vs(42.36±4.64) kg, (16.05±2.18) kg vs(15.29±1.63) kg, (18.62±2.86) kg vs(17.72±2.39) kg, (34.68±4.94) kg vs(33.09±3.61) kg; P< 0.05), but there were no significant differences in FM and FMP between two groups(P> 0.05). Conclusion The FFM in the last trimester in women delivered macrosomic fetus is significantly higher than women delivered normal birth weight infant, but the fat reserves are similar in the two groups.