Despite what we’ve all had drummed into us since the age of ovulation (thanks, mum!) a 28 day cycle isn’t *actually* the norm. In fact, it’s kinda a rarity.

Using the contraceptive app Natural Cycles, researchers analysed data from over 124,648 women across Sweden, USA and the UK. 65 percent of all participants’ cycles fell between the 25-30 day range, but only 13 per cent had a 28 day cycle. Instead, the average was more like 29.3 days.

Admittedly, the difference isn’t extreme – but it is a pretty big deal where fertility is concerned. Firstly, those who track their periods to calculate their ‘safe’ sex window, could be unknowingly putting themselves at risk of pregnancy. And secondly, women who are trying for a baby may be missing their ovulation period.  

“Traditionally, studies have concentrated on women who have approximately 28 day cycles and these studies have formed our understanding of the menstrual cycle,” explained Professor Joyce Harper from the UCL Institute For Women’s Health. “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 also demonstrate that ovulation does not occur consistently on day 14 and therefore it is important that women who wish to plan a pregnancy are having intercourse on their fertile days,” she added. “In order to identify the fertile period, it is important to track other measures.”

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