How well do women understand their menstrual cycles? Insights from 383,085 UK based women

Background: Menstrual health literacy is a critical yet frequently overlooked component of reproductive health; it shapes an individual’s ability to identify abnormal menstrual patterns, seek timely medical care, and make informed decisions. However, many women of reproductive age lack essential knowledge about their cycles, contributing to widespread misconceptions, stigma, and delays in healthcare engagement. This study explores menstrual health awareness, the misclassification of cycle regularity, and demographic disparities among UK-based women. Methods: This was a retrospective, cross-sectional analysis of self-reported menstrual health data collected through an online health assessment (OHA) between September 2020 and January 2025 from 383,085 UK-based women aged 18-50 years. Results: More than one in five participants (22.2%) did not know their cycle length, with younger individuals (under 25) being least aware (33.4% did not know cycle length). Awareness improved with age but declined after 45 years. Ethnic differences were observed, with white participants reporting the highest degree of uncertainty (22.5%). Those who had been pregnant or were trying to conceive showed greater awareness (79.2% and 82.5% respectively). The period length awareness was greater than the cycle length, but 9.5% of participants overall could not report their period length. 13.8% of those under 25 did not know their period length, and awareness improved with age, but slightly declined after 45 years. Hormonal contraceptive users presented the highest uncertainty for both cycle (36.3%) and period length (14.5%). Among those who self-reported regular cycles, 4.9% had cycle lengths outside the typical 21–35-day range. Misclassification was most common among participants under 25 (7.2%), as well as black (6.4%) and asian (5.5%) participants. Conclusions: These findings highlight persistent gaps in menstrual health literacy, and the urgent need for targeted menstrual health literacy initiatives are needed to mitigate misinformation and empower individuals to recognise and address menstrual health concerns. Addressing these disparities is critical to improving reproductive health outcomes and promoting menstrual equity. Trial registration: Not applicable.