Abstract: ObjectiveTo assess the relationships between pre-pregnancy body mass index(BMI)and the 4 adverse birth outcomes,including small for gestational age(SGA),large for gestational age(LGA),low birth weight(LBW),and macrosomia.MethodsSubjects were 12 355 pregnant women who had joined in the Anhui Birth Defects and Child Development Cohort Study and delivered between October 2008 and October 2010.The relationships between pre-pregnancy BMI and SGA,LGA,LBW and macrosomia were evaluated by Chi-square analysis and non-conditional logistic regression.ResultsThe average pre-pregnancy BMI for the pregnant women was 20.19(2.348 kg/m²).Based on the pre-pregnancy BMI,2 834 women were classified as underweight(22.9%),8 745 as normal(70.8%),and 776 as overweight or obese (6.3%).After adjusted for confounding factors such as age,registered residence,and pregnant times,we figured out the relationship between pre-pregnancy BMI and the gestational age group and the birth weight group.Pregnant underweight increased the risk of SGA(odds ratio〔OR〕=1.320,95% confidence interval〔CI:1.068-1.631〕,pre-pregnant overweight and obesity increased the risks of LGA(OR=1.758,95% CI:1.503-2.056)and macrosomia(OR=1.870,95% CI:1.540-2.271).ConclusionHigh or low pre-pregnancy BMI could effect birth outcomes.