OBJECTIVE: Spontaneous preterm birth is an important cause of neonatal mortality and morbidity. An increasing body of evidence suggests that uteroplacental ischemia plays an important role in the etiology of spontaneous preterm birth. We aimed to study whether antiplatelet agents reduce the risk of spontaneous preterm birth. DATA SOURCES: We included data from an individual participant data meta-analysis of studies that had evaluated the effect of antiplatelet agents to reduce preeclampsia (Perinatal Antiplatelet Review of International Studies Individual Participant Data). METHODS OF STUDY SELECTION: The meta-analysis included 31 studies that randomized women to low-dose aspirin-dipyridamole or placebo-no treatment as a primary preventive strategy for preeclampsia. For the current study we analyzed data from 17 trials (28,797 women) that supplied data on type of delivery (spontaneous compared with indicated birth). TABULATION, INTEGRATION, AND RESULTS: Primary endpoints were spontaneous preterm birth at less than 37 weeks, less than 34 weeks, and less than 28 weeks of gestation. We analyzed outcomes for each trial separately using x2 statistics and combined in an individual participant data meta-analysis using a binary logistic regression model. Women assigned to antiplatelet treatment compared with placebo or no treatment had a lower risk of spontaneous preterm birth at less than 37 weeks (relative risk [RR] 0.93, 95% confidence interval [CI] 0.86-0.996) and less than 34 weeks of gestation (RR 0.86, 95% CI 0.76-0.99). The RR of having a spontaneous preterm birth at less than 37 weeks of gestation was 0.83 (95% CI 0.73-0.95) for women who have had a previous pregnancy and 0.98 (95% CI 0.89-1.09) for women in their first pregnancy. The treatment effect was stable in all other prespecified subgroups. CONCLUSION: Antiplatelet agents reduce spontaneous preterm birth in pregnant women at risk for preeclampsia.