The Average Menstrual Cycle Isn't 28 Days. It's Time We Dispel This Dated Period Myth | Opinion

For generations we have been taught that the average menstrual cycle is 28 days in length, split by ovulation into two roughly equal parts, each 14 days in length. During the first phase, the follicular phase, the egg develops and is ovulated around day 14. The second part of the cycle, the luteal phase, is mainly involved with preparation of the womb for the possible implantation of the embryo.

This conventional wisdom is included in numerous medical text books, on health and wellbeing websites and school sex education programmes.

For women who are trying to get pregnant, it is important to have intercourse around the time of ovulation. The egg is only viable for about 24 hours after ovulation, but sperm can last for five days in the female reproductive tract. Therefore the fertile window lasts for a total of six days, ending on the day of ovulation—but there is a higher chance of getting pregnant the closer intercourse is to ovulation.

Historically, women have used their menstrual cycle dates taking day 14 as the day of ovulation, or more complex methods such as measuring their basal body temperature which increases after ovulation or changes in cervical mucus and lutenizing hormone (LH) surge identification to determine when they are ovulating. But without the help of technology these methods can become time consuming and stressful.

Using an app that can calculate the fertile window using an algorithm may be easier and more accurate. The Natural Cycles app asks women to input their menstrual cycle dates, basal body temperature and optionally their LH information so the algorithm can identify when the woman is ovulating.

Studying 600,000 menstrual cycles of over 120,000 women

Many menstrual cycles tracking apps 'find' the fertile days of the cycle based on the assumption of a 28 day cycle with ovulation on day 14. But experts working within the fertility education space have long suspected that menstrual cycles and fertile days are in fact far more varied—so is the 28 day menstrual cycle a myth?

Our latest research published in Nature Digital Medicine is one of the biggest overviews of menstrual cycles to date and analyzed data from more than 600,000 menstrual cycles from 124,648 women from Sweden, the U.S. and the U.K.

The study was a collaboration between University College London and Natural Cycles. Natural Cycles have developed the only FDA cleared app for contraception and fertility monitoring. Our findings aim to improve our knowledge of the menstrual cycle across the population. It could help couples better understand when pregnancy is likely and unlikely to occur.

We set out to investigate menstrual cycle characteristics and associations with age, BMI and body temperatures. We found that menstrual cycles are considerably varied with only 13 percent of women having cycles that last 28 days.

Scientists tracked the menstrual cycles of over 120,000 women using a fertility app. They found the idea the average woman's cycle is 28 days is a myth. iStock

The findings show an average cycle length is 29.3 days. Across the study, 65 percent of women had cycles that lasted between 25 and 30 days. We found the average follicular phase length was 16.9 days and the average luteal phase length was 12.4 days. The average cycle length decreased by 0.18 days and average follicular phase length decreased by 0.19 days per year of age from 25 to 45 years. The average variation of cycle length for obese women was 0.4 days—or 14 percent higher. Cycle length variability was observed to a lesser extent in non-obese and underweight women.

Traditionally studies have been biased as they 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 also demonstrate that ovulation does not occur consistently on day 14, therefore it is important that women who wish to plan a pregnancy identify their fertile window. To do this, it is important to track other measures such as basal body temperature, as cycle dates alone are not informative.

Given the variations in cycle length and follicular phase length that we have described, especially for cycles outside the average range (25 to 30 days), an individualized approach to identify the fertile window should be adopted. Women who rely on thinking they are ovulating on day 14 of their cycle, and apps giving predictions of fertile days based solely on cycle dates could completely miss the fertile window. It is therefore unsurprising that several studies have shown that calendar apps are not accurate in identifying the fertile window.

Huge potential for new discoveries

Besides the potential benefits to the individual, fertility awareness apps and the associated databases of fertility data provide a unique opportunity to examine a large number of menstrual cycles in order to improve understanding. Even for women trying not to get pregnant, it is useful for them to understand how their individual menstrual cycle works and when they are ovulating.

The main limitation of this paper is that the study population is derived solely from users of the app who may not be representative of the wider population. In particular, only eight percent of women in our study were obese compared to 15 percent of women in the general population.

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.

This is one of the largest ever analyses of menstrual cycle characteristics. These initial results only scratch the surface of what can be achieved. We hope to stimulate greater interest in this field of research for the benefit of public health.

Joyce Harper is Professor of Reproductive Science at University College London in the Institute for Women's Health where she is head of the Reproductive Health Department, Principal Investigator of the Reproductive Science Group, Director of Education and Director of the Centre for Reproductive Health. She is a Director of the Embryology and PGD Academy which she established with Alpesh Doshi in 2014 and founder of Global Women Connected.

Views expressed in this article are the author's own.

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