should be mentioned that in many scientific circles, the 28-day
menstrual cycle is believed to be a result of the impact of the modern
world. Up until the last hundred years, the menstrual cycle is said to
have reacted to seasonal changes. Fertility was at its peak during the
fall months, thus helping guarantee the survival of the newborn during
the much more hospitable spring and summer environment. The
menstrual/fertility cycle could last as long as 90 days, and was absent
during times of serious stress. "