<p>Approximately 1.8 billion people experience hormonal changes caused by the menstrual cycle, which impacts work, school, and health factors like physical activity. Prior research shows high-level insights, such as reduced activity during symptomatic days, but the day-to-day relationship between the menstrual cycle and physical activity remains unclear. We performed a 28-day study monitoring physical activity in 26 healthy, naturally menstruating women using an inertial measurement unit on the shank. A validated model estimated energy expended per step. Physical activity was significantly higher during the early-follicular phase than the late follicular phase due to more steps. Energy expenditure was compared with open-source daily estradiol and progesterone data collected from different study cohorts of healthy, eumenorrheic women. Peak estradiol and physical activity showed a correlation (<i>r</i>^<i>2</i> = 0.64) when activity was shifted by 2 days, suggesting a delayed relationship. Precisely measuring movement with wearable sensors could help uncover menstrual cycle effects on activity and improve health management and workplace policies.</p>