AbstractBackgroundPreeclampsia is a significant contributor to maternal and neonatal morbidity and mortality. Folic acid supplementation is recommended periconceptionally for the prevention of neural tube defects. Epidemiological evidence suggests that maternal folic acid supplementation may play a role in preventing other adverse birth outcomes. This systematic review aimed to investigate the effect of maternal folic acid supplementation during pregnancy on risk of preeclampsia and gestational hypertension.MethodsMultiple scientific databases and grey literature were searched for relevant studies. Studies were reviewed according to pre‐specified inclusion and exclusion criteria. Study characteristics were summarised and study quality was assessed. A meta‐analysis of observational studies was conducted to examine the effect of maternal folic acid supplementation on preeclampsia risk.ResultsMeta‐analysis of eight observational studies showed significantly lower odds of preeclampsia with folic acid supplementation in comparison to no folic acid supplementation: OR = 0.78 (95% CI 0.63, 0.98), with moderately high heterogeneity between studies. Subgroup analysis showed no significant subgroup difference between folic acid supplementation taken by itself, in comparison to folic acid taken in or alongside a multivitamin.ConclusionLow level evidence is available for a modest association between maternal folic acid supplementation and reduction in preeclampsia risk. Future studies should differentiate between early and late onset and mild vs severe preeclampsia, and should control for relevant confounders including the presence of multivitamin supplements.The protocol for this systematic review was prospectively registered with PROSPERO (CRD42015029310).