Objective: Recent study suggested that fetal growth and development are related with prenatal environment including nutrition. And early environmental influences have long term effect on fetal body composition and musculoskeletal development. Several hypotheses have been suggested in terms of undernutrition, energy intake and appetite control. Among these hypotheses, studies on the effects of fat mass and fat free mass on metabolic rate and appetite control are underway. The objective was to identify the association of maternal body composition with small for gestational age (SGA). Methods: In this retrospective cohort study consisted of singleton pregnant women who were assessed for physical activity and body composition at 2nd and 3rd trimester in Seoul national university hospital. Physical activity was assessed by the International Physical Activity Questionnaire (IPAQ). Body composition including Body mass index (BMI), Total body water (TBW), Intra-cellular water (ICW), Extra-cellular water (ECW), Body fat mass (BFM), Fat free mass (FFM), Visceral fat area (VFA) and Basal metabolic rate (BMR) were evaluated with bioelectrical impedance analysis (InBody®, Seoul, Korea). We defined SGA as less than the tenth percentile of a Korean growth reference. Results: During the study period, 113 pregnant women were included. At 2nd trimester of pregnancy, SGA group FFM and BMR were lower in the SGA group. [FFM, 41.6 kg in non-SGA birth group vs 39.0 kg in SGA birth group, p=0.028; BMR, 1299 kcal/day in non-SGA birth group vs 1239 kcal/day in SGA birth group, p=0.043]. This tendency was similarly confirmed in the 3rd trimester of pregnancy especially in FFM of lower extremity. Conclusion: Lower maternal FFM and BMR in second and third trimester of pregnancy confer an increased risk to deliver SGA neonate.