PROJECT SUMMARY (See instructions): Postpartum hemorrhage (PPH), defined as the loss of 1 L of blood or more within 24 hours after birth, is the leading cause of maternal mortality worldwide. Importantly, PPH is the most preventable cause of maternal mortality, and the leading factors causing preventable PPH are delays in diagnosis and treatment. Thus, there is an urgent need for an early and accurate alert system that can facilitate prompt treatment to prevent PPH-related morbidity and mortality. During hemorrhage, the body tries to compensate for blood loss by shunting blood from the periphery to vital organs and replenishing reduced blood volume with water from the interstitium. These compensation mechanisms help stabilize the patient, delaying the time until global vascular indicators such as blood pressure and heart rate are affected, the current measurements used to predict PPH. Thus, monitoring of peripheral blood flow and blood content can yield early indicators of hemorrhage. Optical technologies are well suited to noninvasively measure blood flow and blood content. Preliminary experiments demonstrated sensitivity to reduced perfusion in vivo and measured significant differences between blood samples diluted to physiologic levels seen during PPH using complimentary optical approaches. We hypothesize that early changes in peripheral blood flow and water transfer from interstitium to the vasculature will be detected using wearable optical techniques, and such changes can signal early stages of PPH. We will develop a multifunctional wearable optical device and test it in pregnant patients at high and low risk of PPH. These studies will create a database of PPH signatures that will help inform diagnostic algorithms that will provide an early warning system for improved PPH management.