The average menstrual cycle length is 29 days – but they vary hugely among women, according to the findings of a major global study published today.
The findings suggest that women should avoid counting days to determine their chances of pregnancy, according to researchers at University College, London.
They found that just 65% of women had cycles that last between 25 and 30 days – and just 13% had 28 day cycles.
The findings come from 600,00 menstrual cycles and more than 124,000 women in the UK, Sweden and the USA, reported in Nature Digital Medicine.
The data came from users of an app that uses body temperature to help identify the progress of menstrual cycles.
Obesity increased cycle length by 0.4 days on average, the research found.
Age also affected cycle length. It decreased by 0.18 days per year between the ages of 25 and 45. The follicular phase increased by 0.19 days per year of age.
The average follicular phase length was 16.9 days.
Researcher Professor Joyce Harper said: “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.
“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.”
Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. Nature Digital Medicine 27 August 2019
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