One Size Fits Few


Twenty six percent of the world population menstruates. Despite this, the menstrual cycle is historically understudied. What we think we know about the menstrual cycle is based on dated studies including as few as 10 women.

The menstrual cycle has long been depicted as lasting 28-days with ovulation occurring on day 14. The two figures above show the difference between the standard depiction and what researchers are learning from mobile apps.

The menstrual cycles of 200,000 women using the apps Kindara and Sympto reveals that ovulation only occurs at day 14 in 24% of individuals, meaning that most users ovulate outside the previously defined range, anywhere between day 10 and day 24. In addition, overall menstrual cycle length varies more than the standard definition allows, with only 35% in line with the 28-day marker.

This is a meaningful shift in our understanding, because the menstrual cycle is an important indicator of overall health, beyond tracking periods and family planning.  Small fluctuations in one’s regular cycle, or deviations from the standard cycle, can be a sign of an array of health concerns including chronic diseases and sexually transmitted infection.

Databyte via Symul, L., Wac, K., Hillard, P. et al. Assessment of menstrual health status and evolution through mobile apps for fertility awareness. npj Digit. Med. 2, 64 (2019). Data visualization by Tasha McAbee.