Insights into menstrual cycles gleaned from new UCL, Natural Cycles study

Researchers analysed key characteristics of over 600,000 menstrual cycles and data from nearly 125,000 women from Sweden, the UK and the US using the Natural Cycles app.
By Leontina Postelnicu
10:59 am

There is no shortage of fertility apps out there for women looking to get - or not get - pregnant. However, when it comes to figuring out the best options for fertility, one-size-fits-all might not be the best method. New research out of Nature Digital Medicine found that only 13% of women had menstrual cycles that are 28 days in length. Authors of the large-scale study, published this week, suggest that a personalised approach could better support patients in understanding when pregnancy is likely and unlikely to occur.


The researchers from UCL and Natural Cycles sought out to analyse key characteristics, including associations with age, BMI and body temperature, of over 600,000 menstrual cycles and data from nearly 125,000 women in Sweden, the US and the UK using the company’s contraceptive app between September 2016 and February this year.

The women, aged 18 to 45, with a BMI between 15 and 50, had not been using hormonal contraception within a year prior to registering. Users with a pre-existing medical condition, such as polycystic ovarian syndrome, hypothyroidism or endometriosis, or that had menopausal symptoms were not included in the study.


Their findings show that only 13% of women have a menstrual cycle that lasts 28 days, with the average length being 29.3 days. In the study, 65% of women had menstrual cycles standing between 25 and 30 days. Meanwhile, the average length of the follicular phase – which starts on the first day of menstruation and ends with ovulation - was 16.9 days, and 12.4 days for the luteal phase – after ovulation and before the period.  

“For the first time, our study shows that few women have the textbook 28 day [sic] cycle, with some experiencing very short or very long cycles,” said Professor Joyce Harper, from the UCL Institute for Women’s Health.

“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,” Professor Harper explained.

Therefore, women tracking fertility to either prevent or plan for a pregnancy could benefit from understanding “their own cycle characteristics rather than relying on a standardised cycle”, the researchers wrote.


Natural Cycles was cofounded by Dr Elina Berglund, a former CERN physicist, and her husband Dr Raoul Scherwitzl. It offers a subscription service including an app and a basal thermometer, which allows users to take their temperature in the morning and input information about their cycle for the app’s algorithm to then generate insights about their daily fertility.

The app is CE certified as a contraceptive in Europe, and in 2018 the FDA allowed marketing of the app for use in the US for pre-menopausal women aged 18 and over. The previous year, Natural Cycles raised $30m in a round led by EQT Ventures, with existing backers Sunstone, E-ventures and Bonnier Growth Media also participating.

Controversy also surrounded the app last year, however, as the Swedish Medical Products Agency launched an investigation after it was revealed that 37 out of 668 women that sought an abortion at a major hospital in Stockholm during September and December 2017 had been using the app to prevent a pregnancy.

After assessing reports of unplanned pregnancies among active users of the app over a period of six months, the agency said the number was “consistent with data shown in the clinical evaluation included in the certification documentation” – 93% effective with typical use and 98% effective with perfect use based on an analysis of more than 15,000 women carried out by the company - and closed its investigation.

“Since it is important that a contraception app is correctly used, we requested the manufacturer to clarify the risk of unwanted pregnancies in the instructions for use and in the app. These issues have been addressed by Natural Cycles and thereby our review is completed,” said at the time agency investigator Mats Artursson.


Today’s study, however, is limited to data collected anonymously from app users, and might not be representative of the wider population, the researchers said.

But the potential of using data gathered by apps and wearables that consumers are turning to cannot be ignored, they added.

“The widespread use of mobile phone apps for personal health monitoring is generating large amounts of data on the menstrual cycle. Provided that the real-world data can be validated against traditional clinical studies done in controlled settings, there is enormous potential to uncover new scientific discoveries,” Professor Harper concluded.


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