Objectives: To evaluate the effects of polyphenol-containing products during pregnancy on preeclampsia-related maternal and neonatal outcomes. 

Design: Systematic review and meta-analysis. 

Setting: Nine databases and one trial registry, from inception to August 11th, 2023. 

Population/Sample: Randomised controlled trials where women received polyphenolic-containing products (as standardised extracts or dietary supplements) compared to placebo or standard care. 

Methods: All review stages were conducted by two independent reviewers. Random-effects meta-analysis with the Hartung-Knapp-Sidik-Jonkman method using a framework for studies with few events. Main Outcome Measures: Clinical outcomes combining the core outcome set for preeclampsia and WHO's priority outcomes. 

Results: Fourteen trials investigating six candidates were included. In women with preeclampsia, the addition of epigallocatechin gallate (EGCG) to nifedipine may reduce the time needed to achieve blood pressure control (mean difference (MD) = -14.10 min, 95% CI -18.46 to -9.74) and increase the time to the next hypertensive crisis (MD = 3.10 h, 95% CI 2.35 to 3.85) compared to nifedipine alone (1 trial, 349 women; low certainty). Similarly, the addition of resveratrol to nifedipine may reduce the time needed to achieve blood pressure control (MD = -15.50 min, 95% CI -19.83 to -11.17) and increase the time to the next hypertensive crisis (MD = 2.50 h, 95% CI 2.09 to 2.91) (1 trial, 349 women; low certainty). No differences were observed for other outcomes or candidates (Salvia miltiorrhiza, Bryophyllum pinnatum, raspberry and cranberry extracts). 

Conclusion: ECGC and resveratrol supplements have been investigated for potential effects in managing clinical signs and symptoms of preeclampsia; however, evidence on the clinical and adverse effects of polyphenols is limited and uncertain.