Abstract Body (Do not enter title and authors here): Background
Pulse wave velocity (PWV) is a marker of arterial stiffness and cardiovascular risk. While conventionally measured using tonometry or oscillometric systems, recent advances suggest that photoplethysmography (PPG) derived features correlate with PWV, enabling estimation via wearables such as smart rings.
Maternal cardiovascular adaptations during pregnancy, including systemic vasodilation and increased vascular compliance, follow a U-shaped trajectory. Compliance typically increases until the second trimester and then decreases toward term, a pattern observable through both PPG and PWV. This provides a test case for PPG-based PWV estimation.
This study evaluates the accuracy of PPG-derived PWV (ePWV) and its ability to capture pregnancy-related physiological changes when monitored with a smart ring.
Methods
Carotid-femoral PWV (cf-PWV) was measured simultaneously with Oura Ring PPG using the SphygmoCor XCEL system in a separate cohort of 300 healthy individuals. A model was trained on 200 participants using wearable-derived features and evaluated in the remaining 100.
The model was then applied to an anomymised pregnancy dataset from 5,664 users of the third-generation Oura Ring who self-reported pregnancy. Night-time PPG signals were collected during sleep from one month before conception to six months postpartum. For each participant, a baseline was defined from the month preceding conception, and ePWV deltas were calculated as relative changes. Delta values were aligned to the observation period.
Results
Validation of the ePWV algorithm against cf-PWV showed strong agreement (r = 0.76), with a mean difference of 0.210 m/s and SD of 0.999 m/s.
ePWV followed the expected U-shaped pattern during pregnancy. A gradual decline was observed after conception, reaching a minimum near the end of the second trimester, followed by a steady rise into the third trimester. Additionally, ePWV peaked one week postpartum before returning to pre-pregnancy levels within six months.
Conclusion
PPG-derived ePWV showed strong agreement with cf-PWV reference values enabling monitoring of arterial stiffness using wearable smart ring. In addition, the ePWV patterns captured pregnancy-related vascular adaptations, reproducing the U-shaped trajectory described in prior studies. These findings support the feasibility of wearables for scalable non-invasive monitoring of arterial stiffness in response to physiological adaptations.