BACKGROUND AND AIM: The prevention of abnormal fetal growth can improve the health status of children during infancy and in the future. However, the effect of folic acid supplementation on fetal weight for gestational age is unclear. OBJECTIVE: To investigate the effect of folic acid supplementation during pregnancy on abnormal fetal growth. METHODS: This study was an observational study based on a baseline survey of the Jinan birth cohort. Multivariate logistic regression models were constructed to analyze the effects of folic acid supplementation on the risk of abnormal fetal growth and the stratified analyses were used to assess effects of folic acid supplementation in different subgroups. RESULTS: A total of 6501 pairs of mothers and single live births met inclusion and exclusion criteria in the 6640 pairs baseline population of a birth cohort. The proportion for appropriate-for-gestational-age (AGA), Small-for-gestational-age (SGA), and Large-for-gestational-age (LGA) were 73.0 %, 5.2 %, and 21.7 % (1413/6501) among the study population. For the delivery of SGA, cumulative folic acid supplementation for more than 4 months was a protective factor in the mothers who were <
35 years of age (OR = 0.76, 95 % CI: 0.58-0.99) or primiparas (OR = 0.73, 95 % CI: 0.54-0.98). While, for the delivery of LGA, folic acid supplementation during pregnancy was also a protective factor among the mothers who had 13 years of educational time or more (OR = 0.69, 95 % CI: 0.51-0.94). CONCLUSION: Cumulative supplementation of folic acid for more than 4 months may be appropriate to reduce the risk of SGA for mothers aged <
35 years and primiparas. Continuing to take folic acid supplements after the first trimester may increase the risk of LGA for multiparas and women with lower education time.