Objective: To investigate the correlation between maternal weight gain rates in different trimesters and fetal birth weight. Methods: 440 normal women were selected. Maternal age, gravidity, parity, weight before pregnancy were collected at the first time of registering; maternal weight at 12 weeks was recorded retrospectively. Measure and write down maternal weight at 28 weeks and weight before delivery. Follow up fetal birth weight, gestational age and the sex of the baby. The correlation between maternal weight gain rate of different trimesters and fetal birth weight was analyzed by pearson, s correlation. Logistic regression analyse the effection of maternal characters on macrosomia. Result: Maternal weight gain rate in the second trimester is quickest, 0.51 g/week; the first trimrster is 0.01 g/week, slowest; 0.46 g/week of the later trimester. There are statistical differences among them( p<0.05). The incidence of macrosomia was increased with the growing of gestation age, highest in later term. Pre-pregnancy weight, total weight gain, and maternal weight gain rate were significantly related to the birth weight. Conclusion: Maternal weight gain rate in the second trimester is quicker than the first and last trimester. The most sensitive period of maternal weight gain for the birth weight was the second trimester. Furthermore, the incidence of large for date infant is increased with the increasing of gestational age.