New Study Proves The 28-Day Menstrual Cycle Is A Myth

In 1968 Gloria Steinem wrote a satirical essay called "If Men Could Menstruate". It imagined a world in reverse, one in which men had periods and the womb’s monthly shedding was a symbol of power and not of weakness.

"To prevent monthly work loss among the powerful, Congress would fund a National Institute of Dysmenorrhea," she wrote. "Doctors would research little about heart attacks, from which men would be hormonally protected, but everything about cramps."

Today, Steinem’s exercise in doublethink is as relevant as it ever was. We can grow human body parts in petri dishes but we still know relatively little about women’s reproductive health. For instance, studies which look at erectile dysfunction outnumber ones which look at PMS (premenstrual syndrome) by five to one. When you think about the fact that PMS is thought to affect 80% of women of childbearing age while around 50% of men in their 30s were recently found to have experienced difficulty maintaining an erection, the obvious disparity is clear.

Perhaps this shouldn’t come as a surprise to us. After all, it’s well documented that there is a gender health gap, both in terms of the fact that women make up 70% of the world’s health workforce but only 30% of its leadership and that relatively few studies on women’s hormones have been funded, which is why we don’t know how many women are actually adversely affected by hormonal contraception.

This is, in part, because women were historically excluded from clinical trials (this is well documented, perhaps best by Professor Carolyn Mazure at Yale and Angela Saini in her recent book Inferior) and, in part, because drug companies haven’t wanted to fund research into women’s reproductive health.

It’s a testament to how little we know about our periods that today the contraceptive app Natural Cycles has released new data, in partnership with UCL, which shows that only 13% of women actually have a 28-day menstrual cycle.

Do you remember your PHSE lesson at school? Do you remember being told about the textbook 28-day cycle? Do you remember internalising the idea that anything other than that was 'abnormal'?

Well, this study, published today in Nature Digital Medicine is one of the biggest overviews of menstrual cycles to date. Because of the app’s reach, it was able to analyse data from over 600,000 menstrual cycles and 124,648 women from Sweden, USA and the UK.

The findings show an average cycle length is 29.3 days and only around 13% of cycles are 28 days in length. Across the study, 65% of women had cycles that lasted between 25 and 30 days. This is hugely significant and will contribute to a greater understanding of fertility and ovulation.

As Professor Joyce Harper from UCL Institute for Women’s Health puts it: "Our study is unique in analysing over half a million cycles and re-writing our understanding of the key stages. Traditionally studies have concentrated on women who have approximately 28-day cycles and these studies have formed our understanding of the menstrual cycle."

"For the first time, our study shows that few women have the textbook 28 day cycle, with some experiencing very short or very long cycles. We studied all women who used the app," she added. Data was collected from women using the app between September 2016 and February 2019 aged 18 to 45 with a BMI between 15 and 50, who had not been using hormonal contraception within the 12 months prior to registration and did not have a pre-existing medical condition such as polycystic ovarian syndrome, hypothyroidism or endometriosis.

Natural Cycles has had its problems. There have been questions raised over its efficacy and reports of unwanted pregnancies. But you could argue that one of the reasons women have turned to apps like Natural Cycles for contraception or period trackers like Clue in the first place is because of how little we know about our bodies. Knowledge is power and women can’t make informed choices without information. Drug companies have always funded medical research, so what’s the difference if contraceptive apps are stepping in to fill in the gaps where they left off?