Background: Folic acid supplementation (FAS) has been shown to prevent neural tube abnormalities. However, the association between maternal FAS and fetal development outcomes beyond the twelfth week of pregnancy is still unclear. This cohort study aimed to explore the effects of maternal FA intake prior to and during pregnancy on the incidence of small for gestational age (SGA), large for gestational age (LGA), low birth weight (LBW), and high birth weight (HBW).

Patients and Methods: A total of 505 pregnant women who visited Semnan health centers between 2018 and 2021 participated in this present prospective Semnan birth cohort study. Data regarding FAS intake was recorded in different periods. Univariate comparisons employed t-tests, one way analysis of variance (ANOVA), and chi-square tests. To control for confounders, multivariable logistic regression and general linear models (ANCOVA) were used, adjusting for maternal age, BMI, education, employment, and residence. Adjusted odds ratios and mean differences with 95% CIs were calculated.

Results: We found that ongoing FAS (400 µg/day) in 1 month before and all three trimesters notably enhanced the risk of HBW (adjusted OR = 2.95, 95% CI: 1.02–8.52) without affecting on the incidence of SGA, LBW, and LGA.

Conclusions: To verify these results, it is recommended to conduct further studies or large-scale trials along with monitoring folate levels during the entire pregnancy.