Menstrual cycle shortest and longest cycle lengths is considered very irregular. The average menstrual cycle lasts 28 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 25 to 39. Subsequently, the variability increases slightly for women aged 40 to 44. The luteal phase of the menstrual cycle is about the same length in most individuals (mean 14.13 days, standard deviation 1.41 days) whereas the follicular phase tends to show much more variability (log-normally distributed with 95% of individuals having follicular phases between 10.3 and 16.3 days).
Menstrual synchrony of northwestern California, according to one ethnographic study, "all of a household's fertile women who were not pregnant menstruated at the same time..." The phenomenon of menstrual synchrony is the closeness in time of the menstrual cycle onsets of two or more women. The phenomenon is not synchronization in the strict sense of concordance of menstrual cycle onsets but the term "menstrual synchrony" is still used perhaps misleadingly. As an undergraduate, Martha McClintock published the first study on menstrual synchrony; her report detailed the menstrual synchrony of undergraduate women living in a dormitory in Wellesley College. Since then, there have