Pre-eclampsia is associated with deficient intravascularproduction of prostacyclin, a vasodilator, and excessiveproduction of thromboxane, a vasoconstrictor and stimulant ofplatelet aggregation. These observations led to the hypotheses thatlow dose aspirin might prevent or delay development of preeclampsia. To assess the effectiveness and safety of low dose aspirinfor women at risk of developing pre-eclampsia. Comprehensive searching of Pubmed and the Cochranelibrary for randomized controlled trials of low dose aspirin inprevention of pre-eclampsia. The search was last updated inJanuary 2009.The search was done using the following terms: aspirin,low dose aspirin, acetyle salicylic acid, pre-eclampsia, preeclampsia, pregnancy induced hypertension, prevention of preeclampsia,primary prevention of pre-eclampsia, low dose aspirinand pre-eclampsia, aspirin and preeclampsia, low dose aspirin andhigh risk pregnancy. All randomised trials comparing low dose aspirin witheither placebo or no treatment agent were included. Participantswere pregnant women at risk of developing pre-eclampsia.Interventions were any comparisons of low dose aspirin witheither placebo or no treatment. Statistical analyses are performed using the reviewmanager software (RevMan 5). Results are presented as summaryof risk ratio within 95% confidence interval. Number needed totreat is calculated using StatsDirect software and is again within95% confidence interval. Overall there is a 6% reduction in the risk of pre-eclampsiaassociated with the use of low dose aspirin. The included 12trials, with 27,308 women show, RR 0.94, (95% CI 0.86 to 1.02),RD -0.62% (95% CI -0.01% to 1.22%) and NNT 163 (95% CI 81.7 to 9468.6). Aspirin has no effect in prevention of eclampsiaRR 1.00 (95% CI 0.6, 1.66). Low dose aspirin did not reduce theincidence of gestational hypertension RR 1.03 (95% CI 0.93,1.14). Preterm delivery in the included studies is defined asdelivery less than 37 completed weeks. The statistical analysisshows 7% reduction in the incidence of preterm deliveries. RR0.93 (95% CI 0.89, 0.98), RD -1.83% (95% CI -0.93% to 2.72%),NNT 55 (95% CI 37, 107). The results are statistically significantas P value is 0.006. Low dose aspirin had a greater effect on theincidence of preterm deliveries among the participants whoentered the study at > 20 weeks gestation (RR 0.92 VS RR 0.95). Low-dose aspirin has moderate benefits when used forprevention of pre-eclampsia and its consequences. Thisinformation should be discussed with women at risk of preeclampsiato help them make informed choices about theirantenatal care. Whether individual women will choose to takeaspirin might depend on an assessment of their absolute risk.From a public-health perspective, especially for populationswith a high risk of pre-eclampsia, even these moderate benefitscould make more widespread use of aspirin worthwhile.