Abstract: Objective:To investigate the effects of body mass index(BMI) before pregnancy and different periods of pregnancy on neonatal body mass.Methods:The data of the complete prenatal records of 2 260 singleton pregnant women with normal delivery were retrospectively analyzed.Results:The incidence rates of macrosomia in obese and overweight groups were significantly higher than that in low and normal weight groups during pre-pregnancy and early pregnancy(P<0.01).During the second and third trimester,the incidence rate of macrosomia in obese groups was the highest,and the incidence rate of macrosomia in low and normal weight groups was the lowest,the difference of which was not statistically significant(P<0.01).Conclusions:The BMI before pregnancy and different periods of pregnancy is positive correlation with mocrosomia.Strict monitoring the pregnant BMI and controlling weight growth in the normal range are beneficial to prevent the occurrence of macrosomia,and the health of mothers and children.