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Study of 383,085 women in the UK found that over 1 in 5 could not report their menstrual cycle length. Among women under 25, this rose to about 1 in 3.

Menstrual cycle awareness is a critical yet frequently overlooked component of reproductive health; it shapes an individual’s ability to recognise changes in their cycle, seek timely medical care when needed, and make informed decisions. However, many women of reproductive age lack essential understanding about their cycles, contributing to widespread misconceptions, stigma, and delays in healthcare engagement. This study explores menstrual cycle awareness, the perceptions of cycle regularity, and demographic disparities among UK-based women. This was a retrospective, cross-sectional analysis of self-reported menstrual cycle data collected through an online health assessment (OHA) between September 2020 and January 2025 from 383,085 UK-based women aged 18–50 years. Participants reported cycle regularity, cycle length, and period length, alongside demographic and reproductive characteristics. Overall, 22.2% of participants reported not knowing their cycle length, with uncertainty highest among those under 25 years (33.4%). Awareness increased with age before declining slightly after age 45. Awareness of period length was higher than cycle length awareness; however, 9.5% of participants were unable to report period length, rising to 13.8% among those under 25. Among participants who described their cycles as regular and reported a cycle length, 4.9% reported cycle lengths outside commonly referenced regular range (21–35 days), indicating discordance between perceived regularity and reported cycle length. This discordance was most pronounced among younger participants and varied modestly across ethnic groups. Participants who had previously been pregnant or were actively trying to conceive demonstrated higher cycle length awareness. These findings highlight persistent gaps in menstrual cycle awareness among UK-based women, particularly among younger individuals and those not actively engaged in fertility planning. Discordance between perceived cycle regularity and cycle length reference ranges suggests that many individuals may lack a clear framework for interpreting their menstrual patterns. Improving population-level education and clinical engagement around menstrual cycle awareness beyond fertility-focused contexts may support earlier recognition of potential menstrual health concerns and more timely healthcare engagement.

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