Abstract: Objective: To investigate the effects of maternal age and pre-pregnancy body mass index(BMI) on the birth weight of newborns. Methods: Five hundred and thirty-seven primipara with singleObjective: Ton fetus were divided inObjective: To low age group (144 cases),appropriate age group(289 cases), older age group(80 cases) and advanced age group(24 cases) according Objective: To the maternal age， and obesity group(9 cases), over weight group(44 cases),normal weight group(416 cases) and over light group(68 cases) according Objective: To the pre-pregnancy BMI. The birth weight and incidence of macrosomia of newborns in all group were compared. Results: The difference of neonatal weight between low age group and advanced age group was not statistical significance(P ＞ 0. 05), the neonatal weight in appropriate age group and older age group were significantly higher than that in low age group(P ＜ 0. 01). The neonatal weight in over weight group and obesity group were significantly higher than that in normal weight group(P ＜ 0. 01), the neonatal weight in over light group was significantly lower than that in normal weight group(P ＜ 0. 01). Conclusions: The maternal age and prepregnancy BMI are related Objective: To the birth weight of newborns.