Heterogeneous effects of physical activity on physiological stress during pregnancy

Pregnancy involves rapid physiological and psychological changes that can increase vulnerability to health complications, underscoring the need for timely, individualized support. Mobile health (mHealth) tools offer a scalable way to capture repeated measures of health status throughout pregnancy, facilitating longitudinal assessment and the opportunity for timely intervention. This study leveraged mHealth technologies, including the Oura smart ring and ecological momentary assessment (EMA) via a mobile app, to examine how emotional distress affects the relationship between physical activity (PA) and heart rate variability (HRV), an indicator of physiological stress during pregnancy. Specifically, we examined whether emotional distress, measured via daily EMA surveys, moderates the association between physical activity and nighttime HRV, captured by continuous Oura ring data. Hence, this analysis integrated temporally aligned wearable and self-report data to investigate the interaction between subjective emotional states and objectively measured physical activity patterns. Consenting participants, aged 18-40 years, with a healthy singleton pregnancy in the second trimester, were enrolled in the study. Our findings revealed that on days with high emotional distress, each additional 1,000 steps was associated with a 3.5% increase in nighttime HRV (p-value < 0.001; 95% CI: 2.6%, 4.4%). In contrast, physical activity had little to no association with HRV on days with moderate distress (0.6%; 95% CI: -0.7%, 1.9%) and low distress (0.6%; 95% CI: -0.4%, 1.5%). These findings suggest that physical activity may be particularly beneficial on high-distress days, supporting the development of adaptive interventions that prioritize PA engagement during periods of elevated emotional distress. Based on our model-estimated moderation effects, we may recommend that a pregnant woman increase her physical activity on high-distress days due to a strong positive PA-HRV association, whereas for those who do not experience much emotional distress, the recommendation may be less emphasized, given the weaker observed association.