Antenatal Platelet Response on Aspirin: a Pharmacokinetic Study Through Pregnancy

Inclusion Criteria: * \<16 weeks' gestational age * Singleton pregnancy * Plan to take 81mg aspirin due to high risk history (below), but not yet initiated * ≥1 risk factor: * Chronic hypertension * Type I or II diabetes * Previous preeclampsia * Renal disease * Autoimmune disease (SLE) OR ≥2 risk factor: * Nulliparity * IVF pregnancy * Black race or socioeconomic disadvantaged * BMI\>30 * Prior adverse pregnancy outcome Exclusion Criteria: * Contraindication to aspirin * Current or planned use of any other anticoagulation * Current need for dialysis * Use of aspirin therapy prior to enrollment in the current pregnancy * Thrombocytopenia (\<150) * Other known platelet disorder/thrombophilia at enrollment