Check out the work of Beverly Strassmann, a scientist who studied menstruation and related social customs in "ancestral" societies. Her findings, in terms of lifetime menstrual cycles of females, is that 40 years worth of 28-day cycles is not the human norm. I forget her exact numbers, but they are along the lines of a modern-day four-fold increase in menstrual cycles when compared to women before the industrial age - i.e., for the 400 lifetime menses of today's woman, earlier woman had 100 lifetime menses. She takes into account later menarche, frequent pregnancy, and prolonged breastfeeding, but she also reports that non-pregnant and non-nursing women average much-longer-than 28-day cycles, averaging 7 menstrual cycles per year. This type of statistic becomes significant when it is correlated with lower rates of "female" diseases.
