Abstract: Objective:To analyze the effects of maternal body mass index(BMI) on pregnant women and newborn.Methods:Nine hundred pregnant women were divided into the lean group (BMI<20 kg/m²),normal group(20kg/m² ≤ BMI<25kg/m²) and obesity group(BMI ≥ 25 kg/m²) according to the BMI before pregnancy.The pregnant women were divided into the less than 15kg group,15 to 20kg group and more than 20kg group according to the body mass increasing degree during pregnancy.The effects of maternal BMI before pregnancy and body mass increasing on the delivery mode and outcome of pregnant women were recorded.Results:The spontaneous labor rate and cesarean section rate in lean group,normal group and obesity group decreased and increased successively,respectively(P<0.01).With the increasing of body weight of pregnant women,the cesarean section rate and macrosomia birth rate showed an upward trend(P<0.01).The detection rates of gestational hypertension,gestational diabetes,labor block,postpartum hemorrhage and neonatal asphyxia in lean group,normal group and obesity group increased gradually(P<0.01).With the increasing of body mass during pregnancy,the detection rates of gestational hypertension,gestational diabetes,postpartum hemorrhage in obesity group increased gradually(P<0.01).Conclusions:Obesity and excessive increasing weight during pregnancy can improve the risk of gestational hypertension and diabetes,which also can significantly increase the cesarean section rate,fetal macrosomia occurrence,and adverse pregnancy outcomes(such as labor block,postpartum hemorrhage and neonatal asphyxia).