Objective : The purpose of this study was to determine the independent factors that predict neonatal birthweight and find the relationship between maternal weight gain and neonatal birthweight in women with normal glucose tolerance (NGT) and gestational diabetes mellitus (GDM). Methods : Forty-six women with GDM and one hundred fifty women with NGT were included in the study. All subjects had singleton pregnancies and no medical diseases that may affect the fetal growth and were certain of gestational age by early ultrasonography. Maternal weight at each prenatal visit was recorded and neonatal anthropometic measurement was done within 2 days of birth. Results : The average rate of weight gain (kg/week) in NGT was lowest during the first trimester (0.09±0.10), peaked during the second trimester (0.52±0.14), and slowed after 34 gestational weeks (0.46±0.26). In women with GDM, the average rate of weight gain was also lowest during the first trimester (0.18±0.23), but it was twofold higher compared with women with NGT. There was a significant decrease of the rate of weight gain after 28 gestational weeks in women with GDM. Total weight gain during pregnancy was 3.4 kg less in women with GDM. Neonatal birthweight was correlated with maternal weight gain and the rate of weight gain during 14-27 and 28-33 weeks in NGT. However, birthweight was correlated with maternal weight gain and the rate of weight gain during the first trimester and 14-27 weeks in GDM. Conclusion : This result suggests that the women with GDM who have greater weight gain during the first and the second trimester have a increased risk of excessive fetal growth. Thus strict glycemic control during pregnancy is needed especially in these women.