This is an unblinded randomized controlled Phase I/II trial comparing high risk singleton pregnancies randomized to 162mg daily (daily dose) vs 81mg q12hours (split dose). Participants will be enrolled prior to 16 weeks gestation. The primary outcome is platelet inhibition as assessed by PFA-100 epinephrine closure time, assessed 2-4 weeks after initiation and again at 28-32 weeks gestation. A subset of participants will be enrolled in a pharmacokinetic study to evaluate pharmacokinetics of aspirin in pregnancy at the two dosing intervals. Secondary outcomes include urine thromboxane at each visit, platelet associated microRNAs. Individual factors associated with aspirin pharmacokinetics and pharmacodynamics in pregnancy will be assessed. Finally, the relationship between these pharmacodynamic markers and pregnancy outcome will be evaluated.
