How Frequently Do Women Menstruate?

It's often quoted that typically 28 days pass between the start of one period and the start of the next, but this is only a rule of thumb and may not really that average for our species.  Anything between 21 and 40 days is considered entirely usual.


It is also quite common for cycle length to vary from one month to the next, perhaps only by a day but perhaps by much longer.   Women's cycles are most likely to get into a strict 28 day pattern if they live in groups together.  Women living in the same household frequently report that their periods all synchronise.  This effect can even happen between women in the same workplace, especially if they work very long hours (10 or more hours in a shift).  


Japanese women have the longest cycles on average (every 29-30 days) among women in industrialised countries.  Menstrual cycle length outside of "Western" countries tends to be longer, still.