To estimate the effect of low-dose aspirin started in early pregnancy on the incidence of preeclampsia and intrauterine growth restriction (IUGR). A systematic review and meta-analysis were performed through electronic database searches (PubMed, Cochrane, Embase). Randomized controlled trials of pregnant women at risk of preeclampsia who were assigned to receive aspirin or placebo (or no treatment) were reviewed. Secondary outcomes included IUGR, severe preeclampsia and preterm birth. The effect of aspirin was analyzed as a function of gestational age at initiation of the intervention (16 weeks of gestation or less, 16 weeks of gestation or more). Thirty-four randomized controlled trials met the inclusion criteria, including 27 studies (11,348 women) with follow-up for the outcome of preeclampsia. Low-dose aspirin started at 16 weeks or earlier was associated with a significant reduction in preeclampsia (relative risk [RR] 0.47, 95% confidence interval [CI] 0.34-0.65, prevalence in 9.3% treated compared with 21.3% control) and IUGR (RR 0.44, 95% CI 0.30-0.65, 7% treated compared with 16.3% control), whereas aspirin started after 16 weeks was not (preeclampsia: RR 0.81, 95% CI 0.63-1.03, prevalence in 7.3% treated compared with 8.1% control; IUGR: RR 0.98, 95% CI 0.87-1.10, 10.3% treated compared with 10.5% control). Low-dose aspirin started at 16 weeks or earlier also was associated with a reduction in severe preeclampsia (RR 0.09, 95% CI 0.02-0.37, 0.7% treated compared with 15.0% control), gestational hypertension (RR 0.62, 95% CI 0.45-0.84, 16.7% treated compared with 29.7% control), and preterm birth (RR 0.22, 95% CI 0.10-0.49, 3.5% treated compared with 16.9% control). Of note, all studies for which aspirin had been started at 16 weeks or earlier included women identified to be at moderate or high risk for preeclampsia. Low-dose aspirin initiated in early pregnancy is an efficient method of reducing the incidence of preeclampsia and IUGR.