A survey of more than 383,000 UK women aged 18 to 50 has found that nearly one in four (22%) do not know the length of their menstrual cycle. Among those under 25, the figure rises to one in three. One in 10 respondents could not accurately say how long their period lasted.
The research, published by data-driven women's health platform Hertility, draws on seven years of data and more than half a million health assessments. It highlights a worrying gap in menstrual health knowledge, despite a YouGov poll in March 2024 finding that 51% of women aged 44 and under use an app or device to monitor their cycles.
Disconnect between tracking and understanding
Dr Helen O’Neill, CEO of Hertility and associate professor in reproductive and molecular genetics at UCL, warns that a serious lack of education on the menstrual cycle means red flags regarding health concerns consistently go unchecked. 'Right now, a woman’s cycle lives in one app, her symptoms in another and her hormone results somewhere else,' she explains. 'Each gives her a fragment of information, but not the whole picture, and the burden of interpreting it all has quietly shifted from healthcare systems onto women themselves.'
This is not the first time women in the UK have reported a lack of confidence in understanding their own bodies. In February 2025, another nationwide survey found that only one in 10 women could correctly identify all the signs of heavy menstrual bleeding.
Why do so many women know so little?
Dr O’Neill wants women to be 'as familiar with the phases of their menstrual cycle as they are with the days of the week.' Yet women of all ages, including those in their 50s and 60s, still ask her: 'Why was I never taught the basics about my own body?'
A 2024 study led by the University of Bristol and Anglia Ruskin University found that many young women left school lacking basic menstrual health knowledge, with 62.4% rating their education as 'poor' or 'very poor' at preparing them to manage their periods. Dr O’Neill also notes that many women learn about menstruation from their mothers, sisters, or aunts, which can pass on acceptance of painful or heavy periods as normal.
As part of the report, Hertility asked 45,000 women about their journey to a PCOS diagnosis and found that many had ruled themselves out of seeking care because they did not believe their symptoms were serious enough.
Importance of knowing cycle length
Out of the 383,000 women surveyed, 67.4% fell within the 'clinically regular' 21 to 35 day range. Dr O’Neill explains that the main things to look at are the time between periods, how long the bleeding lasts, and how heavy it is. Irregularities can point to ovulation problems, thyroid dysfunction, PCOS, and other conditions.
Hertility’s report also found that 41% of women actively trying to conceive could not accurately identify their fertile window, meaning four in ten women were trying for a baby without knowing when their bodies were most ready to conceive.
Language and taboo
Dr O’Neill hopes the report will encourage women to feel empowered to understand their bodies and talk openly about them. She argues that common terms such as 'menstrual waste' and 'sanitary products' reinforce the idea that periods are shameful. 'The words we use to describe female bodies contributes to a societal taboo around talking about our reproductive function,' she says. 'It’s also a very insidious way of planting a seed at an early age. In many ways, you’re telling a young girl that she’s not clean.'
