ABSTRACT Hypertensive disorders of pregnancy are the most common medical condition affecting pregnancy and a leading cause of maternal morbidity and mortality in the Unites States. Hypertensive disorders of pregnancy increase lifetime cardiac disease risk in women and infants exposed during pregnancy and are considered an independent, gender-specific cardiovascular risk factor by the American Heart Association. Current interventions to prevent hypertensive disorders of pregnancy are extremely limited and minimally effective. Mindfulness-based interventions hold significant promise as a non-pharmacological intervention to prevent hypertensive disorders of pregnancy; mindfulness-based interventions significantly reduce blood pressure in adults with hypertension and prehypertension. However, prenatal clinical trials of mindfulness-based interventions have excluded women at risk for hypertensive disorders of pregnancy from participating, and have not examined effects of mindfulness on maternal cardiovascular parameters. Our pilot RCT of prenatal mindfulness training for women at risk for hypertensive disorders of pregnancy demonstrated medium to large effects on maternal cardiovascular parameters of risk for hypertensive disorders of pregnancy, including lower ambulatory blood pressure and accelerated fetal growth velocity. However, the mechanisms explaining effects of prenatal mindfulness training on indicators of risk for hypertensive disorders of pregnancy are unknown. Building upon these promising preliminary findings, the proposed RCT will harness subjective and objective ecological momentary assessment (EMA) methodologies (in vivo repeated assessments) in combination with wearable biosensor technology to capture rich epochs of ecologically-valid psychological (Aim 1), physiological (Aim 2), and interpersonal (Aim 3) processes through which mindfulness training may lead to improved maternal cardiovascular parameters and reduced risk for hypertensive disorders of pregnancy. N=150 pregnant women at risk for HDP will be randomized to an 8-week phone-delivered mindfulness intervention or usual care. For every participant, we will measure maternal cardiovascular parameters (24-hour blood pressure and uterine artery resistance values by ultrasound Doppler) before and after the RCT. All participants will complete EMA for 2 weeks ‘bursts’ before and after the RCT to evaluate mechanisms of mindfulness training on maternal cardiovascular parameters. EMA will include smartphone-app based experience sampling of psychological processes; smartphone-app based ambient audio sampling (i.e. the Electronically Activated Recorder [EAR] method) and wearable wrist-worn biosensor monitoring (heart rate and heart rate variability) of physiological responses to everyday experiences. Results will provide new insights into 1) effects of mindfulness training on cardiovascular parameters in pregnancy, 2) pathophysiological mechanisms of hypertensive disorders of pregnancy, ...