It's seen as common knowledge that the 'normal' menstruation cycle lasts for 28 days, or at least that's what we've all been told.
According to a new study conducted by researchers at the University College of London (UCL) in partnership with contraceptive app Natural Cycles, 28-day cycles aren't actually the norm.
In fact, only 13 per cent of women have cycles that last 28 days.
The study, published in Nature, analysed data from more than 610,000 period cycles from 124,648 women from the UK, Sweden and the US, between the ages of 18 and 45, who had a BMI (body mass index) between 15 and 50 and discovered the average length of a woman's menstrual cycle actually lasts 29.3 days, with 65 per cent of cycles falling between 25 and 30 days in length.
While this may not seem like much of a difference, the discovery is actually quite significant, as it contributes to a better understanding of fertility and ovulation – with many couples wanting to conceive likely planning around the most fertility window that's based around the '28-days is normal' cycle – along with refuting the traditional understanding that cycles either side of the 28-days are 'abnormal'.
Talking about the study Professor Joyce Harper from UCL's Institute for Women's Health says, "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."
Adding, "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.
"In order to identify the fertile period, it is important to track other measures such as basal body temperature as cycle dates alone are not informative."
So what does this tell us? Well apart from showing there is still a lot not known about women's reproductive health, it also means that if your cycles last a little longer or a little less than 28 days there's no need to worry.
If you're trying to get pregnant, grab a thermometer to check your basal temperature as you can't always rely on cycle dates.
Is there any point when you should be worried about your cycles though?
Naturopathic doctor Jolene Brighten tells Mind Body Green, "Some women cycle every 26 days or every 32 days, and that is their normal. What matters more is what YOUR normal cycle is," she explains.
However, she says the regularity of your period matters more than the actual length, so if you're not regularly getting your period, or it becomes unpredictable, that's when you should see your doctor.
That being said, if your cycles are very short or very long, that may indicate an underlying health issue Brighten explains.
"If your cycle is 21 days or less, this could indicate a luteal phase defect and insufficient progesterone.
"If your cycles are longer than 35 days, it's a good idea to discuss it with your doctor as this can be a sign of thyroid disease, androgen excess, and in some cases PCOS."
It's promising (albeit a little frustrating that it seems well overdue) that there are more studies being undertaken to better understand women's reproductive health, and this recent study is a good example of how 'textbook' ideas need to be re-examined.
As the study's authors wrote, "This analysis details variations in menstrual cycle characteristics that are not widely known, yet have significant implications for health and wellbeing."
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