Aspirin is recommended in high risk patients to reduce the risk of preeclampsia and preterm birth, which are leading causes of both maternal and neonatal morbidity and mortality, but up to 20% will have these adverse outcomes despite therapy. Gaps in knowledge regarding pregnancy specific aspirin pharmacology and the relationship of aspirin response and pregnancy outcome, along with a lack of consensus on aspirin dosing has limited the effective use of this intervention. The investigators aim to apply principles of clinical pharmacology to determine how to optimally utilize this low cost medication to improve maternal/child health outcomes. This is a Phase I/II randomized controlled trial of high risk pregnancies recommended aspirin; participants will be randomized to take aspirin either 162mg once daily, or 81mg twice a day. Outcomes evaluated will include the difference in aspirin response between these two dosing regimens, the individual factors that impact aspirin pharmacology in pregnancy, and evaluate markers or aspirin response that may be associated with pregnancy outcome.

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.

Inclusion Criteria

• Singleton gestation gestational age <16 0/7 weeks, dating confirmed with ultrasound
• ≥1 high risk factor for preeclampsia or ≥2 moderate risk factors as per United States Preventative Services Task Force (2021)
• Recommendation for 162mg aspirin daily in pregnancy
• Age 16-55 years old

Exclusion criteria

• Contraindication to aspirin
• Current or planned use of any other anticoagulation
• Thrombocytopenia, other known platelet or bleeding disorder
• Abnormally elevated baseline PFA-100 epinephrine closure time prior to aspirin initiation

Data will be submitted to NICHD DASH and can be requested through NICHD DASH

1 year after study completion

Data will be submitted to NICHD DASH and may be requested through DASH

Biostatistical analysis, all laboratory assessments will be conducted by personnel blinded to study groupl