Objective To investigate the association between periconceptional folic acid supplementation and small for gestational age (SGA) birth based on maternal pre-pregnancy body mass index (BMI) and provide evidence for the development of comprehensive prevention programs on SGA birth.Methods Between March,2012 and September,2016,a total of 8 523 pregnant women delivering in the First Affiliated Hospital of Shanxi Medical University were surveyed to collect the information about their demographic characteristics,folic acid supplementation before and during pregnancy and about their infants.Among their infants,1 066 were small for gestational age (case group),7 457 were appropriate for gestational age (AGA) (control group).Unconditional logistic regression model was used to evaluate the association between periconceptional folic acid supplementation and SGA birth in the context of different pre-pregnancy BMI.Results The overall incidence of SGA birth was 12.51％ (1 066/8 523).After adjusting the confounding factors,prepregnancy BMI＜ 18.5 kg/m2 was a risk factor for SGA birth (OR=1.22,95％ CI:1.01-1.47),prepregnancy BMI≥24.0 kg/m2 was associated with a reduced risk of SGA birth (OR=0.81,95％CI:0.68-0.97).After adjusting confounding factors,periconceptional folic acid supplementation was a protective factor for SGA birth (OR=0.82,95％CI:0.68-0.98).After stratified by pre-pregnancy BMI,periconceptional folic acid supplementation was associated with the reduced risk of SGA birth in overweight group (24.0 kg/m2≤BMI＜28.0 kg/m2) with OR of 0.55 (95％CI:0.36-0.85).No significant association was observed in other groups.When examined by folic acid supplement type,periconceptional single folic acid supplementation (400 μg per tablet) was a protective factor for SGA birth (OR=0.82,95％ CI:0.69-0.99).After stratified by pre-pregnancy BMI,periconceptional single folic acid supplementation (400 μg per tablet) was associated with the reduced risk of SGA birth in overweight groups (OR =0.56,95 ％ CI:0.36-0.86).No association was observed between periconceptional folic acid containing multivitamin supplementation and SGA birth.Conclusions Periconceptional folic acid supplementation (400 μg) was associated with reduced risk of SGA birth in women with prepregnancy BMI≥24.0 kg/m2 and ＜28.0 kg/m2.No association between folic acid supplementation and SGA was observed in other groups.This study suggests that pre-pregnancy BMI might modify the influence of folic acid supplementation on the risk of SGA birth.