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.