To explore the relationships between resting energy expenditure (REE) according to indirect calorimetry, fetal biometric parameters by ultrasound, and birthweight in gestational diabetes mellitus (GDM). Sixty-five women with GDM and 60 in the control group were enrolled. The REE, birthweight, and fetal biometric parameters according to ultrasound, including biparietal diameter, head circumference, abdominal circumference (AC), and femur length, were measured. The AC at 29 to 32 weeks and 37 to 40 weeks was larger in the GDM than in the control group (P < 0.01), birthweight was higher in the GDM than in the control group (P < 0.01), and women in the GDM group had higher REE than those in the control group at all stages of pregnancy (P < 0.01). In the control group, all fetal biometric parameters were correlated with birthweight at 37 to 40 weeks (r = 0.418, 0.678, 0.741, and 0.635 for biparietal diameter, head circumference, AC, and femur length, respectively, P < 0.05); however, in the GDM group, only AC was correlated with birthweight at 37 to 40 weeks (r = 0.707; P < 0.05). In the GDM group, REE was correlated with birthweight at all three stages of pregnancy (r = 0.369, 0.381, and 0.446 for 21 to 24, 29 to 32, and 37 to 40 weeks, respectively, P < 0.05), and REE was correlated with AC at 37 to 40 weeks (r = 0.431; P < 0.05). REE is correlated with birthweight in women with GDM from the middle to the end of pregnancy. REE by indirect calorimetry might be potential index for medical nutrition therapy in GDM.