Condensation: Aspirin given at <11 weeks' gestation in high risk women does not reduce the risk of preeclampsia and gestational hypertension but may reduce the risk of preterm delivery. Short title: Early aspirin administration and preeclampsia PROSPERO registration number: CRD42019125006 AJOG at a Glance: Why was this study conducted? • To perform a systematic review and meta-analysis to evaluate the effect of lowdose aspirin initiated at <11 weeks' gestation on the risk of preeclampsia, gestational hypertension, or any hypertensive disorder of pregnancy. Secondary outcomes included preterm delivery at <37 weeks' gestation and fetal growth restriction. Key findings • The administration of low-dose aspirin at <11 weeks' gestation in women with a history of recurrent pregnancy loss, women who had undergone in vitro fertilization or women with thrombophilia or antiphospholipid syndrome was associated with a non-significant decrease in the risk of preeclampsia, gestational hypertension, and any hypertensive disorder of pregnancy. • Early low-dose aspirin reduced the risk of preterm delivery but had no impact on the risk of fetal growth restriction. • Except for preterm delivery and any hypertensive disorder of pregnancy, sensitivity analysis demonstrated similar observations; confirming the robustness of our analysis. What does this add to what is known? • Administration of low-dose aspirin at <11 weeks' gestation to high risk women does not reduce the risk of preeclampsia, gestational hypertension, any hypertensive disorder of pregnancy and fetal growth restriction but might reduce the risk of preterm delivery at <37 weeks of gestation. ABSTRACT OBJECTIVE DATA: Pre-conception or early administration of low-dose aspirin might improve endometrial growth, placental vascularization and organogenesis. Most studies have evaluated the potential benefit of pre-conception or early administration of low-dose aspirin in women with a history of recurrent pregnancy loss, women who have undergone in vitro fertilization or women with thrombophilia or antiphospholipid syndrome. These women are at an increased risk of placenta-associated complications of pregnancy, including preeclampsia, preterm delivery and fetal growth restriction. STUDY: We performed a systematic review and meta-analysis to evaluate the effect of low-dose aspirin initiated at <11 weeks' gestation on the risk of preeclampsia, gestational hypertension, or any hypertensive disorder of pregnancy. Secondary outcomes included preterm delivery at <37 weeks' gestation and fetal growth restriction. STUDY APPRAISAL AND SYNTHESIS METHODS: We searched in MEDLINE via PubMed, EMBASE, Cochrane Central Register of Controlled Trials (CENTRAL), ClinicalTrials.Gov and the World Health Organization International Clinical Trials Registry Platform (WHO-ICTRP) from 1985 to November 2018. Entry criteria were randomized controlled trials evaluating the effect of aspirin administered at <11 weeks' gestation in preventing preeclampsia and/or hypertensive disorders in pregnancy or improvi...