https://www.sciencedaily.com/releases/2019/08/190826191924.htm

The study, published today in npj Digital Medicine, is one of the biggest overviews of menstrual cycles to date and analysed data over 600,000 menstrual cycles and 124,648 women from Sweden, USA and the UK and could help couples better understand when pregnancy is likely and unlikely to occur.

Researchers set out to investigate menstrual cycle characteristics and associations with age, BMI and body temperatures. The findings show an average cycle length is 29.3 days and only around 13% of cycles are 28 days in length. Across the study, 65% of women had cycles that lasted between 25 and 30 days.