Aspirin Supplementation for Pregnancy Indicated Risk Reduction In Nulliparas (ASPIRIN)

Background: Preterm birth (PTB) remains the leading cause of neonatal mortality and long term disability throughout the developed and developing world. Though complex in its origins, a growing body of evidence suggests that first trimester administration of low dose aspirin (LDA) holds promise to reduce the rate of PTB substantially. Hypothesis: The investigators' primary hypothesis is that nulliparous women with no more than two previous first trimester pregnancy losses who are treated with LDA daily beginning between 6 0/7 weeks and 13 6/7 weeks gestational age (GA) through 36 0/7 weeks GA will reduce the risk of preterm birth from all causes. Study Design Type: Prospective randomized, placebo-controlled, double-blinded multicenter clinical trial (patient level 1:1). Population: Nulliparous women between the ages of 18 (or local age of majority) and 40 with no more than two previous first trimester pregnancy losses or any second trimester spontaneous pregnancy loss, a singleton pregnancy between 6 0/7 weeks and 13 6/7 weeks GA confirmed by ultrasound, and no contraindications to aspirin. Other medical conditions, such as sickle-cell anemia, may be considered a contraindication per the judgment of the site investigator. Intervention: Daily administration of low dose (81 mg) aspirin \[also known as acetylsalicylic acid (ASA)\], initiated between 6 0/7 weeks and 13 6/7 weeks GA and continued to 36 0/7 weeks GA compared to an identical appearing placebo. Compliance and outcomes will be assessed biweekly. Outcomes: The primary outcome is to determine whether daily LDA initiated between 6 0/7 weeks and 13 6/7 weeks and continued to 36 0/7 weeks reduces the risk of preterm birth (birth prior to 37 0/7 weeks of pregnancy) by 20%. This will be determined based on assessed date of delivery in comparison to the projected estimated date of delivery, independent of whether or not the preterm delivery is indicated or spontaneous. Secondary outcomes include: * Preeclampsia and eclampsia (hypertensive disorders of pregnancy) * Small for gestational age * Perinatal mortality Other secondary outcomes of interest are: Maternal outcomes: * Vaginal bleeding * Antepartum hemorrhage * Postpartum hemorrhage * Maternal mortality * Late abortion * Change in maternal hemoglobin * Preterm, preeclampsia Fetal outcomes: * Preterm birth \<34 0/7 weeks of pregnancy * Birth weight \<2500g and \<1500g * Fetal loss * Spontaneous abortion * Stillbirth * Medical termination of pregnancy