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).
Tzolk'in agricultural cycle of highland Guatemala, which is also about 260 days. Aveni notes that "the average duration between successive halves of the eclipse season, at 173 ½ days, fits into the tzolkin in the ratio of 3 to 2." This may seem contrived, but the Maya did employ the tzolkin to predict positions of Venus and eclipses. Another theory is that the 260-day period is the length of human pregnancy. This is close to the average number of days between the "first missed" menstrual period and birth, unlike Naegele's rule which is 40 weeks (280 days) between the "last" menstrual