Abstract: Objective To investigate changes of body composition of pregnant women with different level of pre-pregnant body mass index (pBMI) in the second and third trimester. Methods Prospective cohort study design was adopted in this study.The free fat mass (FFM), muscular mass (MM), fat mass (FM), fat mass percentage (FMP), total body water (TBW), extracellular water (ECW) and intracellular water (ICW) were analyzed by multi-frequency bioelectrical impedance analysis.The body composition and its increases in the last two trimesters were compared among different pBMI groups including thin group (n=202, pBMI＜ 18.5 kg/m²), normal group (n=738, 18.5 kg/m²≤pBMI＜ 24 kg/m²), overweight group (n=117, 24 kg/m²≤pBMI＜ 28 kg/m²) and obesity group (n=32, pBMI≥28 kg/m²). Results The changes of FFM, MM, ECW, ICW and TBW showed no significant difference in the four groups (P＞ 0.05).Thegrowth of FM in the obesity group was (2.46±4.34) kg which was significantly lower than other groups (P＜ 0.05).There was significant difference in the increase of FMP in four groups and it reduced with the increase of pBMI level (P＜ 0.01). Conclusion Thegrowth of FFM in the last two trimesters shows no differences in 4 groups, however the FMP changes are obviously different in the pregnant women with different pBMI.