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 12 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.