Abstract: ObjectiveTo explore the effect of pregnant women's body mass index(BMI)on the mode of delivery and the birth weight of infant.Methods360 cases of primiparas of single bearing were selected and grouped according to pre-pregnancy BMI and prenatal BMI respectively:33 cases of pre-pregnancy obesity group and 327 cases of normal pre-pregnancy weight group;204 cases of prenatal obesity group and 156 cases of normal prenatal weight group.With questionnaire, the pregnancy outcome and the birth weight of infant were investigated and compared.ResultsThe mode of delivery and birth weight of infanthad no significant differences between pre-pregnancy obesity group and normal pre-pregnancy weight group(P>0.05);The increasing incidence of cesarean section and gaining of birth weighthad significant differences between prenatal obesity group and normal prenatal weight group(P<0.05);There was a positive correlation between the weight of prenatal gaining and the birth weight of infant(P<0.05).ConclusionStrengthening perinatal health care and pregnancy weight control can lower complications of mother and infant.