Menstrual Cycle - Length

Length

Although many people believe the average menstrual cycle takes 28 days, a large study of more than 30,000 cycles from more than 2300 women showed that the mean cycle length was 29.1 with a standard deviation of 7.5 days and a 95% prediction interval of between 15 and 45 days. In that study, the subset of data with cycle lengths between 15 and 45 days had an average length of 28.1 days with a standard deviation of 4 days. A smaller study of 140 women performed in 2006 found a mean cycle length of 28.9 days.

The variability of menstrual cycle lengths is highest for women under 25 years of age and is lowest, that is, most regular, for ages 35 to 39. Subsequently, the variability increases slightly for women aged 40 to 44. Usually, length variation between eight and 20 days in a woman is considered as moderately irregular menstrual cycles. Variation of 21 days or more is considered very irregular.

A 1979 study of 305 women found that approximately one-third of the subjects had lunar period cycles in length, i.e., a mean cycle length of 29.5 days plus or minus 1 day. Almost two-thirds of the subjects started their cycle in the brighter half of the lunar cycle, significantly more than would be expected by random distribution. Another study found a statistically significant number of menstruations occurred around the new moon.