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 has a cycle onset 14 days before A's next onset. The next time both of them have menstrual cycle onsets, B will have a cycle onset 12 days before A. B will continue to gain two days on A until their onsets coincide, then their cycles will begin to diverge again. The cycles of A and B will repeatedly converge and diverge creating the appearance of synchrony during convergence. This is a mathematical property of cycles of different frequencies and not due to the interaction of A and B. If, in addition, the duration of menstruation is considered (typically 3