Background: Oxidative stress has been proposed as a key factor involved in the development of pre-eclampsia. Supplementing women with antioxidants during pregnancy may help to counteract oxidative stress and thereby prevent or delay the onset of pre-eclampsia.

Objectives: To determine the effectiveness and safety of any antioxidant supplementation during pregnancy and the risk of developing pre-eclampsia and its related complications.

Search strategy: We searched the Cochrane Pregnancy and Childbirth Group Trials Register (June 2004) and the Cochrane Central Register of Controlled Trials (The Cochrane Library, Issue 3, 2004).

Selection criteria: All randomised and quasi-randomised trials comparing one or more antioxidants with either placebo or no antioxidants during pregnancy for the prevention of pre-eclampsia, and trials comparing one or more antioxidants with another, or with other interventions.

Data collection and analysis: Two review authors independently assessed trials for inclusion, data extraction and trial quality. Data were double-entered into the Review Manager software.

Main results: Seven trials involving 6082 women are included in this review. The largest trial (5021 women) was quasi-random and only three of the seven included trials were rated high quality. Supplementing women with any antioxidants during pregnancy compared with control or placebo was associated with a 39% reduction in the risk of pre-eclampsia (relative risk (RR) 0.61, 95% confidence intervals (CI) 0.50 to 0.75, seven trials, 6082 women). Women receiving antioxidants compared with control or placebo also had a reduced risk of having a small-for-gestational-age infant (RR 0.64, 95% CI 0.47 to 0.87, three trials, 634 women), their infants had a greater mean birthweight (weighted mean difference 91.83 g, 95% CI 11.55 to 172.11, three trials, 451 women), but they were more likely to give birth preterm (RR 1.38, 95% CI 1.04 to 1.82, three trials, 583 women). There were insufficient data for reliable conclusions about possible effects on any other outcomes.

Authors' conclusions: These results should be interpreted with caution, as most of the data come from poor quality studies. Nevertheless, antioxidant supplementation seems to reduce the risk of pre-eclampsia. There also appears to be a reduction in the risk of having a small-for-gestational-age baby associated with antioxidants, although there is an increase in the risk of preterm birth. Several large trials are ongoing, and the results of these are needed before antioxidants can be recommended for clinical practice.